This avoids the issue that build breaks when both gtk3
and gstreamer-0.10 are enabled, caused by commit
34bbf192
("Drop extra define ENABLE_GTKSINK").

gstreamer-0.10 related code will be removed in
a subsequent step according to ESC decision (no longer
needed).

Change-Id: Ief07a797a3e52229da0ff7b23478108aac900841
Reviewed-on: https://gerrit.libreoffice.org/73608
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>