Add extsources to libsdl_mixer (#567)

packages/l/libsdl_mixer/xmake.lua

@@ -11,6 +11,12 @@ package("libsdl_mixer")
         add_versions("2.0.4", "9affb8c7bf6fbffda0f6906bfb99c0ea50dca9b188ba9e15be90042dc03c5ded")
     end
 
+    if is_plat("linux") then
+        add_extsources("pacman::sdl2_mixer", "apt::libsdl2-mixer-dev")
+    elseif is_plat("macosx") then
+        add_extsources("brew::sdl2_mixer")
+    end
+
     add_deps("libsdl")
 
     add_links("SDL2_mixer")