x11/lavalauncher: enable LIBRSVG2 by default after r554733

lavalauncher doesn't directly use glib but inherits via librsvg.
USE_GNOME=librsvg2 already pulls glib20 via USE_GNOME_IMPL.

$ fgrep -nr glib /path/to/lavalauncher

$ fgrep -nr glib /usr/local/include/librsvg-2.0
/usr/local/include/librsvg-2.0/librsvg/rsvg.h
30:#include <glib-object.h>

$ pkg-config librsvg-2.0 --libs
-L/usr/local/lib -lrsvg-2 -lm -lgio-2.0 -lgdk_pixbuf-2.0 -lgobject-2.0 -lglib-2.0 -lintl -lcairo -lpthread