vlc: fix glx support

glx support is provided by the mesa3d package; so, enable it only when
mesa3d is part of the package selection.

Fixes:
  http://autobuild.buildroot.org/results/9b4/9b46021ea0064850ce8f1adcb5455d6da5bff459/build-end.log



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>