Fix building glslang on platforms without <filesystem>

[alula]

Certain platforms using older libc++ versions have <filesystem> support disabled, which causes the builds after 7f1d3b1 to fail.

As there's already been a hack added for similar issue on macOS in upstream, this include is unused and can be removed.

[akien-mga]

Makes sense. Could you add that change in the patch file I added to document the changes we made to upstream glslang? This file will be used to reapply the changes next time we update, so if it's not updated we'll reintroduce this include by mistake.

https://github.com/godotengine/godot/blob/master/thirdparty/glslang/glslang/patches/disable-absolute-paths-for-apple-compat.patch

While at it, I notice I put in the wrong place (should be in thirdparty/glslang/patches, not thirdparty/glslang/glslang/patches. So you could move it one folder up.

I can also do it if you're not sure how to tweak the patch to add another section, let me know.

[alula]

done.

[akien-mga]

Thanks!