Separate __has_attribute from defined(__has_attribute)

Fix Visual C warnings:
```
regenc.h(121): warning C4067: unexpected tokens following preprocessor directive - expected a newline
```

Author: nobu

regenc.h

@@ -118,8 +118,10 @@ typedef struct {
 
 typedef struct {
   short int len;
-#if defined(__has_attribute) && __has_attribute(nonstring)
+#if defined(__has_attribute)
+# if __has_attribute(nonstring)
   __attribute__((nonstring))
+# endif
 #endif
   const UChar name[6];
   int       ctype;