add a missing zlib pkgconfig path.

component/xorg/buildout.cfg

@@ -250,7 +250,7 @@ recipe = slapos.recipe.cmmi
 url = http://www.x.org/releases/X11R7.7/src/everything/libXfont-1.4.5.tar.bz2
 md5sum = 6851da5dae0a6cf5f7c9b9e2b05dd3b4
 environment =
-  PKG_CONFIG_PATH=${fontsproto:location}/lib/pkgconfig:${libfontenc:location}/lib/pkgconfig:${xproto:location}/lib/pkgconfig:${xtrans:location}/share/pkgconfig:${freetype:location}/lib/pkgconfig
+  PKG_CONFIG_PATH=${fontsproto:location}/lib/pkgconfig:${libfontenc:location}/lib/pkgconfig:${xproto:location}/lib/pkgconfig:${xtrans:location}/share/pkgconfig:${freetype:location}/lib/pkgconfig:${zlib:location}/lib/pkgconfig
   PATH=${pkgconfig:location}/bin:%(PATH)s
   CPPFLAGS=-I${zlib:location}/include
   LDFLAGS=-L${bzip2:location}/lib -Wl,-rpath=${bzip2:location}/lib -L${zlib:location}/lib -Wl,-rpath=${zlib:location}/lib