smp_call_function_single(): be slightly less stupid, fix #2

fix m68k build failure:

 tip/kernel/up.c: In function 'smp_call_function_single':
 tip/kernel/up.c:16: error: dereferencing pointer to incomplete type
 make[2]: *** [kernel/up.o] Error 1

Signed-off-by: Ingo Molnar <mingo@elte.hu>