x86/efi: Add a prototype for efi_arch_mem_reserve()

... in order to fix a -Wmissing-ptototypes warning:

  arch/x86/platform/efi/quirks.c:245:13: warning:
  no previous prototype for ‘efi_arch_mem_reserve’ [-Wmissing-prototypes] \
	  void __init efi_arch_mem_reserve(phys_addr_t addr, u64 size)

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200326135041.3264-1-b.thiel@posteo.de