unlzma: fix build of stubdom unsafe decompressors

c/s b683d68c38 changed the packing attribute on struct lzma_header.  However,
this is 3rd library code used by stubdomains.

Revert the change to lzma_header alone to avoid needless divergence from its
source.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/common/unlzma.c

@@ -214,11 +214,11 @@ rc_bit_tree_decode(struct rc *rc, uint16_t *p, int num_levels, int *symbol)
  */
 
 
-struct __packed lzma_header {
+struct lzma_header {
 	uint8_t pos;
 	uint32_t dict_size;
 	uint64_t dst_size;
-};
+} __attribute__((packed)) ;
 
 
 #define LZMA_BASE_SIZE 1846