Fix avcodec compile errors

gcc -Wall -ggdb -falign-functions=2 -I. -O2 -DNDEBUG -ffunction-sections -I/usr/include/SDL -D_GNU_SOURCE=1 -D_REENTRANT -Wno-unused-result -DEMU_F68K -D_USE_CZ80   -c -o platform/common/mp3_libavcodec.o platform/common/mp3_libavcodec.c
platform/common/mp3_libavcodec.c: In function ‘mp3dec_start’:
platform/common/mp3_libavcodec.c:97:40: warning: ‘enum CodecID’ declared inside parameter list
  AVCodec *(*avcodec_find_decoder)(enum CodecID id);
                                        ^
platform/common/mp3_libavcodec.c:97:40: warning: its scope is only this definition or declaration, which is probably not what you want
platform/common/mp3_libavcodec.c:141:31: error: ‘CODEC_ID_MP3’ undeclared (first use in this function)
  codec = avcodec_find_decoder(CODEC_ID_MP3);
                               ^
platform/common/mp3_libavcodec.c:141:31: note: each undeclared identifier is reported only once for each function it appears in
platform/common/mp3_libavcodec.c:141:31: error: type of formal parameter 1 is incomplete
<builtin>: recipe for target 'platform/common/mp3_libavcodec.o' failed
make: *** [platform/common/mp3_libavcodec.o] Error 1
notaz / picodrive

Fix avcodec compile errors #26
