Shut up the warning "implicit declaration of strlen".

util-linux/switch_root.c

@@ -9,6 +9,7 @@
 #include <dirent.h>
 #include <fcntl.h>
 #include <stdio.h>
+#include <string.h>
 #include <sys/mount.h>
 #include <sys/stat.h>
 #include <sys/types.h>