meson.build: disable need for OpenGL Window systems
Fixes:
Message: No OpenGL Window systems found or requested gst-libs/gst/gl/meson.build:948:2: ERROR: Problem encountered: GStreamer OpenGL integration required via options, but needed dependencies not found.
With the following configuration:
-Dgl=enabled -Dgl_api='gles2' -Dgl_platform='egl' -Dgl_winsys=''