libs: g-i: avoid multiple libraries in the shared-library tag

libs: g-i: avoid multiple libraries in the shared-library tag

Using multiple libraries causes problems for the C# bindings and
will for similiar languages such as Java when there are bindings
for them.

Also change --library=libgstfoo-X.la to --library=gstfoo-X as
the man page suggests it should be done.

https://bugzilla.gnome.org/show_bug.cgi?id=679315