Use full initializer for initrd_ctx to avoid fatal warnings with older GCC

struct ... foo = { 0, } is valid initializer, but older GCC emits
warning which is fatal error due to -Werror=missing-field-initializer.
So simply use full initializer to avoid these errors. This was fixed
probably in GCC 4.7.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36750

grub-core/loader/sparc64/ieee1275/linux.c

@@ -373,7 +373,7 @@ grub_cmd_initrd (grub_command_t cmd __attribute__ ((unused)),
   grub_addr_t paddr;
   grub_addr_t addr;
   int ret;
-  struct grub_linux_initrd_context initrd_ctx = { 0, };
+  struct grub_linux_initrd_context initrd_ctx = { 0, 0, 0 };
 
   if (argc == 0)
     {