gst-editing-services does not install ges-image-source.h but references it in ges.h

The change in commit ba37e0aa builds fine, but has the result of not installing the ges-image-source.h header. However, the main ges.h references ges-image-source.h, so any project that includes ges.h will fail to build, which is a regression between 1.9.2 and 1.9.3. This caused https://bugzilla.redhat.com/show_bug.cgi?id=2036385 which affects Rygel (among others) in Fedora. The snippet from the build log is:

/usr/include/gstreamer-1.0/ges/ges.h:66:10: fatal error: ges/ges-image-source.h: No such file or directory
   66 | #include <ges/ges-image-source.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~

I am not sure of the best solution, but possibly some header guards in ges.h so that ges-image-source.h is only included during the GIR build, and then installing ges-image-source.h again, would be appropriate.