GLES NDK: Build break fix

- Make sure all headers can be included by themselves.

Change-Id: I727a2ca632f66c6ad39a0df4cf54d7ffde8116a5
diff --git a/ndk/platforms/android-24/include/GLES2/gl2ext.h b/ndk/platforms/android-24/include/GLES2/gl2ext.h
index 2d05596..31b2667 100644
--- a/ndk/platforms/android-24/include/GLES2/gl2ext.h
+++ b/ndk/platforms/android-24/include/GLES2/gl2ext.h
@@ -1,6 +1,8 @@
 #ifndef __gl2ext_h_
 #define __gl2ext_h_ 1
 
+#include <GLES2/gl2.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif