This patch fixes the following error when compiling mplayer with libopus.

libavcodec/libopusdec.c: In function 'libopus_decode_init':
libavcodec/libopusdec.c:130:27: error: implicit declaration of function 'ff_exp10'; did you mean 'ff_exp2fi'? [-Werror=implicit-function-declaration]
         double gain_lin = ff_exp10(gain_db / (20.0 * 256));
                           ^~~~~~~~
                           ff_exp2fi
Signed-off-by: Mikulas Patocka <mikulas@twibright.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>