[efi] Add missing "ULL" suffix on 64-bit constant

Older versions of gcc complain if this suffix is missing.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_watchdog.c

 #define WATCHDOG_TIMEOUT_SECS ( 5 * 60 )
 
 /** Watchdog code (to be logged on watchdog timeout) */
-#define WATCHDOG_CODE 0x6950584544454144
+#define WATCHDOG_CODE 0x6950584544454144ULL
 
 /** Watchdog data (to be logged on watchdog timeout) */
 #define WATCHDOG_DATA L"iPXE";