pkgconfig: fix meson warnings about GL_CFLAGS in gstreamer-gl .pc files

pkgconfig: fix meson warnings about GL_CFLAGS in gstreamer-gl .pc files

We added GL_CFLAGS in autotools because we unconditionally included OpenGL
headers. We do not do that anymore for a standard #include <gst/gl/gl.h>.
OpenGL headers are only included for the platform specific files
e.g. #include <gst/gl/x11/gstgldisplay_x11.h> which must always be hidden
behind a #if GST_GL_HAVE_WINDOW_X11, or #include <gst/gl/gstglfuncs.h> to
gain access to the contents of the GstGLFuncs object.

Users of our headers must include the necessary include paths for the
platforms they are intending to support.

We might introduce extra .pc files for what GstGL was built against though.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/735

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/741>