x86: unify __set_fixmap, fix

fix build failure:

 arch/x86/mm/pgtable.c:280: warning: ‘enum fixed_addresses’ declared inside parameter list
 arch/x86/mm/pgtable.c:280: warning: its scope is only this definition or declaration, which is probably not what you want
 arch/x86/mm/pgtable.c:280: error: parameter 1 (‘idx’) has incomplete type
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/pgtable.c

@@ -2,6 +2,7 @@
 #include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/tlb.h>
+#include <asm/fixmap.h>
 
 pte_t *pte_alloc_one_kernel(struct mm_struct *mm, unsigned long address)
 {