gplugin/gplugin

gi-docgen now errors if your annotations are wrong, so this fixes ours

Testing Done:
Ran the build with docs enabled until it succeeded.

Reviewed at https://reviews.imfreedom.org/r/1131/
debian/tmp/usr/include/gplugin-1.0
debian/tmp/usr/lib/*/libgplugin.so
debian/tmp/usr/lib/*/pkgconfig/gplugin.pc
debian/tmp/usr/share/gir-1.0/GPlugin-1.0.gir
debian/tmp/usr/share/gplugin/valgrind/gplugin.supp
debian/tmp/usr/share/vala/vapi/gplugin.deps
debian/tmp/usr/share/vala/vapi/gplugin.vapi