x86/mm/set_memory: Fix -Wmissing-prototypes warnings

Add missing includes and move prototypes into the header set_memory.h in
order to fix -Wmissing-prototypes warnings.

 [ bp: Add ifdeffery around arch_invalidate_pmem() ]

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200320145028.6013-1-b.thiel@posteo.de