kmscube: fix build on OpenGL ES 3 dependencies not providing GLES3/gl3ext.h

kmscube: fix build on OpenGL ES 3 dependencies not providing GLES3/gl3ext.h

According to Khronos documentation[1], gl2ext.h can be used for OpenGL
ES 3. Some vendor implementation of OpenGL ES 3, such as
Rockchip's libmali[2], do not have a gl3ext.h but only a gl2ext.h while
being OpenGL ES 3 compliant.

This fixes the header files to be included from gl3ext.h to gl2ext.h to
be compatible with (hopefully) all OpenGL ES 3 implementations.

[1] https://www.khronos.org/registry/OpenGL/index_es.php#headers
[2] https://github.com/rockchip-linux/libmali

Cc: Quentin Schulz <foss+yocto@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>