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

Add includes for the prototypes of valid_phys_addr_range(),
arch_mmap_rnd() and valid_mmap_phys_addr_range() in order to fix
-Wmissing-prototypes warnings.

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200402124307.10857-1-b.thiel@posteo.de