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.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>