gst-libav: fix build on systems without C++ compiler

gst-libav: fix build on systems without C++ compiler

Fix the following build failure on systems without C++ compiler:

The following exception(s) were encountered:
Running "/nvmedata/autobuild/instance-9/output-1/host/bin/or1k-buildroot-linux-musl-g++ --version" gave "[Errno 2] No such file or directory: '/nvmedata/autobuild/instance-9/output-1/host/bin/or1k-buildroot-linux-musl-g++'"

Fixes:
 - http://autobuild.buildroot.org/results/8ac0ba5eaaf7571857b4d8cfabf1488d640dc59a

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2112>