Add gcc_struct to all packed structures when compiling with mingw.

Just "packed" doesn't always pack the way we expect.

include/grub/net/udp.h

@@ -27,7 +27,7 @@ struct udphdr
   grub_uint16_t dst;
   grub_uint16_t len;
   grub_uint16_t chksum;
-} __attribute__ ((packed));
+} GRUB_PACKED;
 
 struct grub_net_udp_socket;
 typedef struct grub_net_udp_socket *grub_net_udp_socket_t;