libkvm: Unbreak build for powerpc64/powerpc

This slipped through my testing due to the recent roto-tiling of the
ARCH/TARGET parts that I need to catch up with. Mea culpa.

lib/libkvm/kvm_powerpc64.c

@@ -41,6 +41,7 @@ __FBSDID("$FreeBSD$");
 #include <limits.h>
 #include <kvm.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "kvm_private.h"