Add missing <string.h> includes.

Change-Id: I3d1ad54418f69f947e2d829d63b53d44581bfd86
diff --git a/toolbox/watchprops.c b/toolbox/watchprops.c
index 0d05aba..cd62922 100644
--- a/toolbox/watchprops.c
+++ b/toolbox/watchprops.c
@@ -1,5 +1,6 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <time.h>
 #include <errno.h>