powerpc/lib: Fix randconfig build failure in sstep.c

Under some configs we need to explicitly include cpu_has_feature.h,
otherwise we fail with:

  arch/powerpc/lib/sstep.c:1992:7: error: implicit declaration of function 'cpu_has_feature'
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/sstep.c

@@ -15,6 +15,7 @@
 #include <asm/sstep.h>
 #include <asm/processor.h>
 #include <asm/uaccess.h>
+#include <asm/cpu_has_feature.h>
 #include <asm/cputable.h>
 
 extern char system_call_common[];