build: make the Makefile.am a bit clearer

Something like:
$(INTROSPECTION_GIRS:.gir=.typelib)

is not really clear. Writing "GtkSource-3.0.typelib" is better in my
opinion. It is true that for GtkSourceView 4 there will be more changes,
but "3.0", "3_0" etc are already present at lots of places.