multimedia/ffmpeg: unbreak FLITE=on with Clang after r507592

-Wl,--as-needed is partially broken in base GNU bfd:
/usr/local/lib/libflite_cmu_us_awb.so: undefined reference to `usenglish_init'
/usr/local/lib/libflite_cmu_us_awb.so: undefined reference to `cmu_lex_init'
/usr/local/lib/libflite_cmu_us_kal.so: undefined reference to `cmu_postlex'

PR: 239567
Reported by: VVD