efi/libstub: Fix missing-prototypes in string.c

Fix the following warnings.
drivers/firmware/efi/libstub/string.c:83:20: warning: no previous
prototype for ‘simple_strtoull’ [-Wmissing-prototypes]
drivers/firmware/efi/libstub/string.c:108:6: warning: no previous
prototype for ‘simple_strtol’ [-Wmissing-prototypes]
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1600653203-57909-1-git-send-email-tiantao6@hisilicon.comSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/string.c

@@ -7,6 +7,7 @@
  */
 
 #include <linux/ctype.h>
+#include <linux/kernel.h>
 #include <linux/types.h>
 #include <linux/string.h>