imageworsener: add libpng dependency

Without this, imageworsener is very limited as
it can not write png files

Closes Homebrew/homebrew#15089.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/imageworsener.rb

@@ -5,6 +5,8 @@ class Imageworsener < Formula
   url 'http://entropymine.com/imageworsener/imageworsener-src-0.9.10.tar.gz'
   sha1 'd908a08e3e402052a22390aa865cbb5a6e30d465'
 
+  depends_on :libpng => :recommended
+
   def install
     system "./configure", "--disable-dependency-tracking", "--prefix=#{prefix}"
     system "make install"