Add linux pacman add_extsources for gdk-pixbuf (#875)

packages/g/gdk-pixbuf/xmake.lua

@@ -18,6 +18,8 @@ package("gdk-pixbuf")
         add_extsources("brew::gdk-pixbuf")
         add_syslinks("resolv")
         add_patches("2.42.6", path.join(os.scriptdir(), "patches", "2.42.6", "macosx.patch"), "ad2705a5a9aa4b90fb4588bb567e95f5d82fccb6a5d463cd07462180e2e418eb")
+    elseif is_plat("linux") then 
+        add_extsources("pacman::gdk-pixbuf2")
     end
 
     on_load(function (package)