playlist/cue: support file type declaration "FLAC" (non-standard)

According to http://wiki.hydrogenaud.io/index.php?title=Cue_sheet FLAC
files should use the "WAVE" file type, but I recently encountered CUE
files declared as "FLAC" which could not be read by MPD.

src/playlist/cue/CueParser.cxx

@@ -202,6 +202,7 @@ CueParser::Feed2(char *p) noexcept
 			return;
 
 		if (strcmp(type, "WAVE") != 0 &&
+		    strcmp(type, "FLAC") != 0 && /* non-standard */
 		    strcmp(type, "MP3") != 0 &&
 		    strcmp(type, "AIFF") != 0) {
 			state = IGNORE_FILE;