decoder/fluidsynth: adapt to API change in version 2.0

NEWS

@@ -6,6 +6,8 @@ ver 0.20.22 (not yet released)
   - fix broken command "list ... group"
 * storage
   - curl: URL-encode paths
+* decoder
+  - fluidsynth: adapt to API change in version 2.0
 * Android
   - now runs as a service
   - add button to start/stop MPD

src/decoder/plugins/FluidsynthDecoderPlugin.cxx

@@ -64,7 +64,12 @@ fluidsynth_level_to_mpd(enum fluid_log_level level)
  * logging library.
  */
 static void
-fluidsynth_mpd_log_function(int level, char *message, gcc_unused void *data)
+fluidsynth_mpd_log_function(int level,
+#if FLUIDSYNTH_VERSION_MAJOR >= 2
+			    const
+#endif
+			    char *message,
+			    void *)
 {
 	Log(fluidsynth_domain,
 	    fluidsynth_level_to_mpd(fluid_log_level(level)),