Add gcc_struct to all packed structures when compiling with mingw.

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

grub-core/net/tcp.c

@@ -104,7 +104,7 @@ struct tcphdr
   grub_uint16_t window;
   grub_uint16_t checksum;
   grub_uint16_t urgent;
-} __attribute__ ((packed));
+} GRUB_PACKED;
 
 struct tcp_pseudohdr
 {
@@ -113,7 +113,7 @@ struct tcp_pseudohdr
   grub_uint8_t zero;
   grub_uint8_t proto;
   grub_uint16_t tcp_length;
-} __attribute__ ((packed));
+} GRUB_PACKED;
 
 struct tcp6_pseudohdr
 {
@@ -122,7 +122,7 @@ struct tcp6_pseudohdr
   grub_uint32_t tcp_length;
   grub_uint8_t zero[3];
   grub_uint8_t proto;
-} __attribute__ ((packed));
+} GRUB_PACKED;
 
 static struct grub_net_tcp_socket *tcp_sockets;
 static struct grub_net_tcp_listen *tcp_listens;