webkit2png: add depends_on :macos

Formula/webkit2png.rb

@@ -3,11 +3,15 @@ class Webkit2png < Formula
   homepage "https://www.paulhammond.org/webkit2png/"
   url "https://github.com/paulhammond/webkit2png/archive/v0.7.tar.gz"
   sha256 "9b810edb6f54cc23ba86b0212f203e6c3bbafc3cbdb62b9c33887548c91015bf"
+  license "MIT"
 
   bottle do
     sha256 cellar: :any_skip_relocation, all: "ad209d841f88f9b5d3a969e2493d853237c89234bd09dfc3d1aa2106832d2d7d"
   end
 
+  # Requires Quartz, as well as other potentially Mac-only libraries
+  depends_on :macos
+
   def install
     bin.install "webkit2png"
   end