multimedia/vapoursynth-vmaf: unbreak on FreeBSD < 12 after 97ed9c524d49

FAILED: libvmaf.so
c++ -o libvmaf.so libvmaf.so.p/VMAF_VMAF.cpp.o -flto -flto-jobs=0 -Wl,--as-needed -Wl,--allow-shlib-undefined -Wl,-O1 -shared -fPIC -Wl,--start-group -Wl,-soname,libvmaf.so -fstack-protector-strong /usr/local/lib/libvmaf.so -Wl,--end-group
/usr/bin/ld: unrecognized option '-plugin'
/usr/bin/ld: use the --help option for usage information
c++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

• Error code 1

Reported by: pkg-fallout