package/mplayer: add optional dependencies to libgl, libvpx and opus

package/mplayer: add optional dependencies to libgl, libvpx and opus

Mplayer links to these packages when present:

output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/mplayer | grep NEEDED | grep "opus\|vpx\|GL"
 0x0000000000000001 (NEEDED)             Shared library: [libopus.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libvpx.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libGL.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>