multimedia/aom: unbreak consumers with VMAF=on after r522621

$ make -C /usr/ports/multimedia/mpv
[...]
Checking for libav* is FFmpeg : no
[...]
['cc', 'test.c.1.o', '-o/path/to/mpv-0.31.0/build/.conf_check_*/testbuild/testprog', '-Wl,-Bstatic', '-Wl,-Bdynamic', '-L/usr/local/lib', '-lavcodec', '-fstack-protector-strong', '-fuse-ld=bfd', '-L/usr/local/lib']
err: /usr/local/bin/ld.bfd: /usr/local/lib/libaom.so.0: undefined reference to `fatal'
cc: error: linker command failed with exit code 1 (use -v to see invocation)