x86: set_highmem_pages_init() cleanup, fix !CONFIG_NUMA && CONFIG_HIGHMEM=y

Impact: build fix

 arch/x86/mm/highmem_32.c:187: error: static declaration of 'set_highmem_pages_init' follows non-static declaration
 arch/x86/include/asm/numa_32.h:8: error: previous declaration of 'set_highmem_pages_init' was here

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1236082212.2675.24.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>