Add a header for string.h for strerror and memset

Change-Id: I98dbc5447362f032b3646791c489e1961ddbf9db
diff --git a/tests/jni/android_net_cts_NetlinkSocket.cpp b/tests/jni/android_net_cts_NetlinkSocket.cpp
index 7f430d8..1e671c6 100644
--- a/tests/jni/android_net_cts_NetlinkSocket.cpp
+++ b/tests/jni/android_net_cts_NetlinkSocket.cpp
@@ -21,6 +21,7 @@
 #include <sys/socket.h>
 #include <linux/netlink.h>
 #include <errno.h>
+#include <string.h>
 #include "JNIHelp.h"
 
 #include "android_net_cts_NetlinkSocket.h"