Add gcc_struct to all packed structures when compiling with mingw.

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

grub-core/fs/hfsplus.c

@@ -65,7 +65,7 @@ struct grub_hfsplus_btheader
   grub_uint8_t btree_type;
   grub_uint8_t key_compare;
   grub_uint32_t attributes;
-} __attribute__ ((packed));
+} GRUB_PACKED;
 
 struct grub_hfsplus_catfile
 {
@@ -80,7 +80,7 @@ struct grub_hfsplus_catfile
   grub_uint8_t unused3[44];
   struct grub_hfsplus_forkdata data;
   struct grub_hfsplus_forkdata resource;
-} __attribute__ ((packed));
+} GRUB_PACKED;
 
 /* Filetype information as used in inodes.  */
 #define GRUB_HFSPLUS_FILEMODE_MASK	0170000