x86/mm/32: Fix -Wmissing prototypes warnings for init.c

Fix:

  arch/x86/mm/init.c:503:21:
  warning: no previous prototype for ‘init_memory_mapping’ [-Wmissing-prototypes]
  unsigned long __ref init_memory_mapping(unsigned long start,

  arch/x86/mm/init.c:745:13:
  warning: no previous prototype for ‘poking_init’ [-Wmissing-prototypes]
  void __init poking_init(void)

Lift init_memory_mapping() and poking_init() out of the ifdef
CONFIG_X86_64 to make the functions visible on 32-bit too.

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200606123743.3277-1-b.thiel@posteo.de