My commit 0b792c22a4 was a beginner mistake as pdb/enums.pl is a
generated file. I also need to fix the source otherwise `git diff` will
always differe when generating the file.
Our CI in particular verifies that `git diff` is always empty so this
was breaking our gimp-debian-x64 CI job.
The enums have to be menually registered in pdb/enums-external.pl.
Currently supports enums from GEGL only.
Add enum GeglDistanceMetric as first external enum.