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: Ingo Molnar <mingo@elte.hu>