Add missing include in EGLWindow.cpp

BUG=angleproject:892

Change-Id: I23cfbd9cfbe7d41e048a2b2a4eea048f5de72d12
Reviewed-on: https://chromium-review.googlesource.com/269123
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>

util/EGLWindow.cpp

@@ -4,6 +4,7 @@
 // found in the LICENSE file.
 //
 
+#include <string.h>
 #include <cassert>
 #include <vector>