Add gobject-introspection for ircv3

This includes a pkgconfig file and installs the header files as well as
purpleircv3.h to ${prefix}/include/purple-3/libpurple/protocols/ircv3.

This does not yet include the documentation as I assume that's going to touch
a bunch of files.

Testing Done:
Compiled against gplugin 0.41.0 locally without issue. Also installed to a local prefix and verified everything looked okie dokie.

Reviewed at https://reviews.imfreedom.org/r/2484/