py-matplotlib: depend on pkg-config

On OS X, freetype isn't found by default, but pkg-config can.

var/spack/repos/builtin/packages/py-matplotlib/package.py

@@ -26,6 +26,7 @@ class PyMatplotlib(Package):
     depends_on('py-pbr')
     depends_on('py-funcsigs')
 
+    depends_on('pkg-config')
     depends_on('freetype')
     depends_on('qt', when='+gui')
     depends_on('bzip2')