Update srs_app_ffmpeg.cpp

srs develop 新增支持mp3转码后，需要添加新的acodec-libmp3lame.所以去掉srs_app_ffmpeg.cpp中对acodec的判断

trunk/src/app/srs_app_ffmpeg.cpp

@@ -181,12 +181,15 @@ int SrsFFMPEG::initialize_transcode(SrsConfDirective* engine)
     }
     
     if (acodec != SRS_RTMP_ENCODER_COPY && acodec != SRS_RTMP_ENCODER_NO_AUDIO) {
+        /*
+        // add new acodec libmp3lame
         if (acodec.find(SRS_RTMP_ENCODER_ACODEC) == std::string::npos) {
             ret = ERROR_ENCODER_ACODEC;
             srs_error("invalid acodec, must be %s, actual %s, ret=%d",
                 SRS_RTMP_ENCODER_ACODEC, acodec.c_str(), ret);
             return ret;
         }
+        */
         if (abitrate <= 0) {
             ret = ERROR_ENCODER_ABITRATE;
             srs_error("invalid abitrate: %d, ret=%d", abitrate, ret);