powerpc: Fix idle.c compile warning

This fixes a compile warning when using arch/ppc/kernel/idle.c
in a merged (ARCH=powerpc) kernel.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/ppc/kernel/idle.c

@@ -32,6 +32,7 @@
 #include <asm/cache.h>
 #include <asm/cputable.h>
 #include <asm/machdep.h>
+#include <asm/smp.h>
 
 void default_idle(void)
 {