Add gd dependency to libwmf.

From libwmf's homepage:
    "Export to PNG and JPEG uses Thomas Boutell's GD library."

Formula/libwmf.rb

@@ -4,6 +4,8 @@ class Libwmf <Formula
   url 'http://downloads.sourceforge.net/project/wvware/libwmf/0.2.8.4/libwmf-0.2.8.4.tar.gz'
   homepage 'http://wvware.sourceforge.net/libwmf.html'
   md5 'd1177739bf1ceb07f57421f0cee191e0'
+  
+  depends_on 'gd'
 
   def install
     ENV.libpng
@@ -11,7 +13,6 @@ class Libwmf <Formula
                           "--prefix=#{prefix}",
                           "--with-freetype=/usr/X11"
     system "make"
-
     ENV.j1 # yet another rubbish Makefile
     system "make install"
   end