Fixed global constant declaration (appeared in 9af10e099d09).

This fixes building with GCC 10, which is default to -fno-common.
See: https://gcc.gnu.org/gcc-10/porting_to.html

src/nxt_string.h

@@ -180,7 +180,7 @@ NXT_EXPORT uintptr_t nxt_encode_complex_uri(u_char *dst, u_char *src,
     size_t length);
 NXT_EXPORT nxt_bool_t nxt_is_complex_uri_encoded(u_char *s, size_t length);
 
-NXT_EXPORT const uint8_t  nxt_hex2int[256];
+extern const uint8_t  nxt_hex2int[256];
 
 
 #endif /* _NXT_STRING_H_INCLUDED_ */