libvisual-plugins: only build PortAudio plugin on macOS

Also remove unused `sdl12-compat`

Author: cho-m

Formula/lib/libvisual-plugins.rb

@@ -25,11 +25,13 @@ class LibvisualPlugins < Formula
 
   depends_on "jack"
   depends_on "libvisual"
-  depends_on "portaudio"
-  depends_on "sdl12-compat"
 
   uses_from_macos "bison" => :build
 
+  on_macos do
+    depends_on "portaudio"
+  end
+
   on_linux do
     depends_on "alsa-lib"
     depends_on "mesa"