x86: mm/pgtable_32.c declare set_pmd_pfn before they get used

Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>

include/asm-x86/pgtable_32.h

@@ -31,6 +31,7 @@ static inline void pgtable_cache_init(void) { }
 static inline void check_pgt_cache(void) { }
 void paging_init(void);
 
+extern void set_pmd_pfn(unsigned long, unsigned long, pgprot_t);
 
 /*
  * The Linux x86 paging architecture is 'compile-time dual-mode', it