package/mediastreamer: add optional support for libvpx

When libvpx was compiled before, mediastreamer will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/libmediastreamer_voip.so.3.0.0 | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libmediastreamer_base.so.3]
 0x0000000000000001 (NEEDED)             Shared library: [libopus.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libvpx.so.2]
[...]

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