fix compilation warning: warning: this statement may fall through

```
src/siphash24.c: In function ‘siphash24_compress’:
src/siphash24.c:129:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
                         state->padding |= ((uint64_t) in[6]) << 48;
                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/siphash24.c:130:17: note: here
                 case 6:
                 ^~~~
src/siphash24.c:131:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
                         state->padding |= ((uint64_t) in[5]) << 40;
                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/siphash24.c:132:17: note: here
                 case 5:
                 ^~~~
src/siphash24.c:133:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
                         state->padding |= ((uint64_t) in[4]) << 32;
                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/siphash24.c:134:17: note: here
                 case 4:
                 ^~~~
src/siphash24.c:135:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
                         state->padding |= ((uint64_t) in[3]) << 24;
                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/siphash24.c:136:17: note: here
                 case 3:
                 ^~~~
src/siphash24.c:137:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
                         state->padding |= ((uint64_t) in[2]) << 16;
                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/siphash24.c:138:17: note: here
                 case 2:
                 ^~~~
src/siphash24.c:139:40: warning: this statement may fall through [-Wimplicit-fallthrough=]
                         state->padding |= ((uint64_t) in[1]) <<  8;
                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/siphash24.c:140:17: note: here
                 case 1:
                 ^~~~
```

Author: ssahani

src/siphash24.c

@@ -127,18 +127,25 @@ void siphash24_compress(const void *_in, size_t inlen, struct siphash *state) {
         switch (left) {
                 case 7:
                         state->padding |= ((uint64_t) in[6]) << 48;
+                        /* fall through */
                 case 6:
                         state->padding |= ((uint64_t) in[5]) << 40;
+                        /* fall through */
                 case 5:
                         state->padding |= ((uint64_t) in[4]) << 32;
+                        /* fall through */
                 case 4:
                         state->padding |= ((uint64_t) in[3]) << 24;
+                        /* fall through */
                 case 3:
                         state->padding |= ((uint64_t) in[2]) << 16;
+                        /* fall through */
                 case 2:
                         state->padding |= ((uint64_t) in[1]) <<  8;
+                        /* fall through */
                 case 1:
                         state->padding |= ((uint64_t) in[0]);
+                        /* fall through */
                 case 0:
                         break;
         }