Make EFI_RESERVED_SERVICE a proper prototype

[dim]

Summary

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

In file included from /share/dim/src/freebsd/base/projects/clang400-import/sys/boot/efi/loader/bootinfo.c:43:
In file included from /share/dim/src/freebsd/base/projects/clang400-import/sys/boot/efi/loader/../include/efi.h:52:
/share/dim/src/freebsd/base/projects/clang400-import/sys/boot/efi/loader/../include/efiapi.h:534:32: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
(EFIAPI *EFI_RESERVED_SERVICE) (
                               ^

Let's add VOID to make it into a real prototype. Alternatively, we
could use "...", but I don't think that is necessary.

[imp]

This looks OK. DF fixed this with 'void' instead of 'VOID' FWIW.
Upstream still has this as (), which is clearly wrong.
Also, Upstream is a complicated relationship because upstream has been sliced, diced and fried so completely that's not a concern...

[dim]

In D9132#188924, @imp wrote:

This looks OK. DF fixed this with 'void' instead of 'VOID' FWIW.

VOID is what Windows-like code tends to use. They like their uppercase type names. :-) In the end it's just an alias for void, so it doesn't matter too much.