vlc: select libvorbis when opus is also selected

If BR2_PACKAGE_OPUS is selected then vlc will be built with support for
opus passing "--enable-opus" option to the configure script. But when
the configure script checks for the opus library it also checks for the
ogg library at the same time, so if only opus has been installed and not
ogg, then it will fail with an error message like this one:

checking for OPUS... no
configure: error: Library ogg opus needed for opus was not found

Fixes:
   http://autobuild.buildroot.net/results/4da/4da8e7cfb35c351e5d623835118512248d331d16/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>