arm/linux: Fix ARM Linux header layout

The hdr_offset member of the ARM Linux image header appears at
offset 0x3c, matching the PE/COFF spec's placement of the COFF
header offset in the MS-DOS header. We're currently off by four,
so fix that.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/arm/linux.h

@@ -30,7 +30,7 @@ struct linux_arm_kernel_header {
   grub_uint32_t magic;
   grub_uint32_t start; /* _start */
   grub_uint32_t end;   /* _edata */
-  grub_uint32_t reserved2[4];
+  grub_uint32_t reserved2[3];
   grub_uint32_t hdr_offset;
 };