libsvg-cairo: depend on Homebrew's cairo

We depend on our cairo on all platforms for consistency, but this one
was overlooked.

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

Formula/libsvg-cairo.rb

@@ -8,7 +8,7 @@ class LibsvgCairo < Formula
   depends_on 'pkg-config' => :build
   depends_on 'libsvg'
   depends_on :libpng
-  depends_on :cairo
+  depends_on 'cairo'
 
   def install
     system "./configure", "--disable-dependency-tracking", "--disable-debug", "--prefix=#{prefix}"