MacOSX: PNG Include Was Broken

- wrong var name, should be _DIRS since
  a long time:
    https://github.com/Kitware/CMake/commit/df0f302485c6f93a473e1958830b69b9c165b01a#diff-75b9f97116a34c6419fe4975ac4d6b7b
Merge pull request #42 from ax3l/fix-cmakePNGInclude

MacOSX: PNG Include Was Broken