The prototype of the functions handle_kernel_image & efi_enter_kernel
are defined in efi-stub.c which may result in a compiler warnings if
-Wmissing-prototypes is set in gcc compiler.

Move the prototype to efistub.h to make the compiler happy.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Link: https://lore.kernel.org/r/20200706172609.25965-2-atish.patra@wdc.comSigned-off-by: Ard Biesheuvel <ardb@kernel.org>