meson: Fix invalid include flag in uninstalled gl pc file

${libdir}/gstreamer-1.0/include is only valid after installation, but extra_cflags are added unconditionally, so we can't use that for include flags.

Instead, let's add the include flag via variables, which are different for installed and uninstalled pc files.

This is particularly bad for consuming GStreamer via CMake which barfs on non-existent include paths.

I have tested this by replacing the system gstreamer-gl-1.0.pc