Annotate non NUL-terminated character array

The lookup table in hexchar() is of the size of its number of elements
and thus not implicitly NUL-terminated. Add the nonstring annotation to
please GCC 15:

    ../src/share/hexdecoct.c: In function ‘hexchar’:
    ../src/share/hexdecoct.c:38:39: warning: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
       38 |         static const char table[16] = "0123456789abcdef";
          |                                       ^~~~~~~~~~~~~~~~~~

Author: cgzones

src/share/hexdecoct.c

@@ -35,6 +35,7 @@ int undecchar(char c) {
 }
 
 char hexchar(int x) {
+        __attribute__ ((nonstring))
         static const char table[16] = "0123456789abcdef";
 
         return table[x & 15];