Fix compilation on Linux with SFML_OPENGL_ES option

[kimci86]

Description

Last night in #2157, I introduced without realizing some nonsense in the Window/CMakeLists.txt file.
I hoped CI would catch this kind of mistakes but there is no check with SFML_OPENGL_ES option enabled.

This PR makes it possible to build SFML on Linux with any combination of SFML_USE_DRM and SFML_OPENGL_ES as it was before #2157.

How to test this PR?

cmake -B build -DCMAKE_BUILD_TYPE=Debug -DWARNINGS_AS_ERRORS=False
cmake -B build -DSFML_USE_DRM=False -DSFML_OPENGL_ES=False && cmake --build build
cmake -B build -DSFML_USE_DRM=True  -DSFML_OPENGL_ES=False && cmake --build build
cmake -B build -DSFML_USE_DRM=False -DSFML_OPENGL_ES=True  && cmake --build build
cmake -B build -DSFML_USE_DRM=True  -DSFML_OPENGL_ES=True  && cmake --build build

I had to disable WARNINGS_AS_ERRORS because some OpenGL ES specific parts trigger warnings, but that is a different topic.

Tested on Ubuntu and Raspbian.

[eXpl0it3r]

Thanks for the fix! 🙂