meson: use library() for libgstcheck instead of always building a shared lib

Otherwise we try to build a shared lib when we build the rest
of GStreamer statically, which won't work because we pass
-DGST_STATIC_COMPILATION when building statically, which means
we won't dllimport public symbols from our libs which means
that on Windows the unit tests will fail to link to libgstcheck.

https://bugzilla.gnome.org/show_bug.cgi?id=797185