package/libvpx: vp9 encoder needs c++

package/libvpx: vp9 encoder needs c++

Encoding rate control interface for v9 is implemented in c++:
https://github.com/webmproject/libvpx/commit/745979bc2913b4f26847084699d9fddd68b60410#diff-58f26933c120328b31831a743eec5ba3R534

Disable vp9 encoding when toolchain lacks c++.

Fixes:
http://autobuild.buildroot.net/results/983/9836215fe22f260483f9153fb5447892801a14a9/
http://autobuild.buildroot.net/results/88a/88aa80ee006ba2dcc40c15be3af73c756cb62974/

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