gnome_devel_whitelist: add gtk-mac-integration 2.1.3

This fixes the error produced by `brew audit gtk-mac-integration --online`:

Error: 1 problem in 1 formula detected
gtk-mac-integration:
  * 2.1.3 is a development release

This problem is currently blocking https://github.com/Homebrew/homebrew-core/pull/42536

Library/Homebrew/dev-cmd/audit.rb

@@ -691,6 +691,7 @@ module Homebrew
       gnome_devel_whitelist = %w[
         libart 2.3.21
         pygtkglext 1.1.0
+        gtk-mac-integration 2.1.3
       ].each_slice(2).to_a.map do |formula, version|
         [formula, version.split(".")[0..1].join(".")]
       end