ffmpeg libopusdec: fix missing include file in libopusdec.c

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>

libavcodec/libopusdec.c

@@ -24,6 +24,7 @@
 
 #include "libavutil/internal.h"
 #include "libavutil/intreadwrite.h"
+#include "libavutil/ffmath.h"
 
 #include "avcodec.h"
 #include "internal.h"