Backport of "meson: Generate a pkgconfig file for the GES plugin" into 1.16

This was missing due to a typo.

Should be backported to 1.16 because it's causing a packaging failure in Cerbero:

0:03:55 ***** Error running 'package' command:
0:03:55 The following files required by this package are missing:
 lib/gstreamer-1.0/libgstges.la