libkkc: FTBFS in Fedora rawhide/f31

[fujiwarat]

Originally reported in Fedora 31 bugzilla; https://bugzilla.redhat.com/show_bug.cgi?id=1736028

Seems libkkc build is failed with the latest vala 0.45.3 .

[fujiwarat]

The following is the build log:

BUILDSTDERR: metadata-file.vala:46.9-46.27: error: Creation method of abstract class cannot be public.
BUILDSTDERR:         public MetadataFile (string name, string filename) throws Error {
BUILDSTDERR:         ^^^^^^^^^^^^^^^^^^^
BUILDSTDERR: language-model.vala:104.9-104.28: error: Creation method of abstract class cannot be public.
BUILDSTDERR:         public LanguageModel (LanguageModelMetadata metadata) throws Error {
BUILDSTDERR:         ^^^^^^^^^^^^^^^^^^^^
BUILDSTDERR: make[2]: *** [Makefile:1429: libkkc_la_vala.stamp] Error 1