add missing include

Change-Id: Id93278c59cfdf72a62a4fe26178bd5c3e43b089e
diff --git a/include/system/window.h b/include/system/window.h
index 1cc4a0a..9602445 100644
--- a/include/system/window.h
+++ b/include/system/window.h
@@ -18,6 +18,7 @@
 #define SYSTEM_CORE_INCLUDE_ANDROID_WINDOW_H
 
 #include <stdint.h>
+#include <string.h>
 #include <sys/cdefs.h>
 #include <system/graphics.h>
 #include <cutils/native_handle.h>