freewheeling: build with fluidsynth (#157085)

Fixes constant stream of warning messages
"UserVariable: WARNING: Can't convert invalid variable!"
due to "SYSTEM_fluidsynth_enabled" var in the default
"coreinterface.xml".

Enables MIDI synthesis out of the box.

pkgs/applications/audio/freewheeling/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, pkg-config, autoreconfHook, gnutls, freetype
+{ lib, stdenv, fetchFromGitHub, pkg-config, autoreconfHook, gnutls, freetype, fluidsynth
 , SDL, SDL_gfx, SDL_ttf, liblo, libxml2, alsa-lib, libjack2, libvorbis
 , libSM, libsndfile, libogg, libtool
 }:
@@ -19,7 +19,7 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ pkg-config autoreconfHook libtool ];
   buildInputs = [
-    freetype SDL SDL_gfx SDL_ttf
+    freetype fluidsynth SDL SDL_gfx SDL_ttf
     liblo libxml2 libjack2 alsa-lib libvorbis libsndfile libogg libSM
     (gnutls.overrideAttrs (oldAttrs: {
       configureFlags = oldAttrs.configureFlags ++ [ "--enable-openssl-compatibility" ];