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

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

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

The following exception(s) were encountered:
Running "/home/autobuild/autobuild/instance-0/output-1/host/bin/i686-buildroot-linux-gnu-g++ --version" gave "[Errno 2] No such file or directory: '/home/autobuild/autobuild/instance-0/output-1/host/bin/i686-buildroot-linux-gnu-g++'"

Fixes:
 - http://autobuild.buildroot.org/results/eebf65036f79d21d347714d62afecd0108393308

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2200>