libsysutils: Add string.h to includes

Signed-off-by: San Mehat <san@google.com>
diff --git a/libsysutils/src/NetlinkListener.cpp b/libsysutils/src/NetlinkListener.cpp
index d871341..96a616d 100644
--- a/libsysutils/src/NetlinkListener.cpp
+++ b/libsysutils/src/NetlinkListener.cpp
@@ -17,6 +17,7 @@
 
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <string.h>
 
 #define LOG_TAG "NetlinkListener"
 #include <cutils/log.h>