cairo: always depend on our pixman

Older versions of XQuartz, including the one distributed by Apple with
OS X, contain an insufficient version of pixman for recent cairo.

Potentially the :pixman dep could be taught to only delegate to XQuartz
if it is up to date enough, but until then, always depend on our pixman.

Fixes Homebrew/homebrew#14072.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/cairo.rb

@@ -11,7 +11,7 @@ class Cairo < Formula
 
   # Can't currently pass :recommended to the :x11 dep.
   depends_on :libpng # => :recommended
-  depends_on :pixman
+  depends_on 'pixman'
   depends_on 'pkg-config' => :build
   depends_on 'xz'=> :build
   depends_on :x11 unless ARGV.include? '--without-x'