[retroarch] libretro-flycast should be compiled with GLES

[Shatur]

The core doesn't work because it's compiled to use OpenGL, whereas RA is compiled against OpenGLES.
We need to add -DUSE_GLES=ON to the PKGBUILD. I tried it locally and can confirm that it works. Developers helped me to figure it out, see [url="https://github.com/flyinghead/flycast/issues/1248"]this issue[/url].