Fix C99 compatibility

Author: martinmares

src/ngx_http_auth_jwt_binary_converters.c

@@ -38,12 +38,13 @@ int hex_to_binary( const char* str, u_char* buf, int len )
 	if (odd) {
 		return -1;
 	}
-
-	for (int i = 0; i < len; i += 2) {
+        
+	int i;
+	for (i = 0; i < len; i += 2) {
 		hex_char_to_binary( *(str + i), &high );
 		hex_char_to_binary( *(str + i + 1 ), &low );
 
 		*cpy++ = low | (high << 4);
 	}
 	return 0;
-}
+}