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

arch/x86/include/asm/pgtable.h

@@ -999,15 +999,12 @@ extern int direct_gbpages;
 void init_mem_mapping(void);
 void early_alloc_pgt_buf(void);
 extern void memblock_find_dma_reserve(void);
-
-
-#ifdef CONFIG_X86_64
-extern pgd_t trampoline_pgd_entry;
-
 void __init poking_init(void);
-
 unsigned long init_memory_mapping(unsigned long start,
 				  unsigned long end, pgprot_t prot);
+
+#ifdef CONFIG_X86_64
+extern pgd_t trampoline_pgd_entry;
 #endif
 
 /* local pte updates need not use xchg for locking */