Make EFI_RESERVED_SERVICE a proper prototype

With clang 4.0.0, the EFI API header causes the following warning:

In file included from sys/boot/efi/loader/bootinfo.c:43:
In file included from sys/boot/efi/loader/../include/efi.h:52:
sys/boot/efi/include/efiapi.h:534:32: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
(EFIAPI *EFI_RESERVED_SERVICE) (

^

Add VOID to make it into a real prototype.

Reviewed by: imp, emaste, tsoome
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D9132