util: reinstate ENUM_PACKED

gets rid of warning: 'gcc_struct' attribute ignored [-Wattributes] introduced by !23338

Fixes: 86532fa21d ("util: Use the gcc_struct attribute for packed structures in mingw")
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23478>

src/util/macros.h

@@ -209,8 +209,10 @@ do {                       \
 #  else
 #    define PACKED __attribute__((__packed__))
 #  endif
+#  define ENUM_PACKED __attribute__((packed))
 #else
 #define PACKED
+#define ENUM_PACKED
 #endif
 
 /* Attribute pure is used for functions that have no effects other than their