AudioFileWave: add "LMMS" as software metadata string

By Mikobuntu:

I have created a patch ( tested on stable) that adds a metadata entry
"Software" with the value "LMMS (libsndfile-1.0.21)" I'm not quite sure
what calls in the libsndfile entry as my only field entered was LMMS,
but i guess it does no harm anyway to know ;)

Closes #3374272.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>

src/core/audio/AudioFileWave.cpp

@@ -69,6 +69,7 @@ bool AudioFileWave::startEncoding()
 		default: m_si.format = SF_FORMAT_WAV | SF_FORMAT_PCM_16; break;
 	}
 	m_sf = sf_open( outputFile().toUtf8().constData(), SFM_WRITE, &m_si );
+	sf_set_string ( m_sf, SF_STR_SOFTWARE, "LMMS" );
 	return true;
 }