Allow FindDCMTK.cmake to be re-used directly adding missing module.

Usage of 'find_package_handle_standard_args' was initially introduced
by commit 52d9539f5e7.

For more details about the issue, see http://forum.dcmtk.org/viewtopic.php?f=1&t=3906&p=15599#p15599

See #431
Merge branch '431-easier-re-use-of-finddcmtk'

* 431-easier-re-use-of-finddcmtk:
  Allow FindDCMTK.cmake to be re-used directly adding missing module.