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

Utilities/CMake/FindDCMTK.cmake

@@ -323,6 +323,7 @@ endif()
 # Compatibility: This variable is deprecated
 set(DCMTK_INCLUDE_DIR ${DCMTK_INCLUDE_DIRS})
 
+include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(DCMTK
   REQUIRED_VARS ${DCMTK_INCLUDE_DIR_NAMES} DCMTK_LIBRARIES
   FAIL_MESSAGE "Please set DCMTK_DIR and re-run configure")