COMP: Fix build against ITK install adding "expat_external.h" install…

… rule

This commit fixes a regression introduced in 9e337d3 (ENH: Update expat
files) by pull request #2755 on 2021-09-27.

ITK issue is #3186 and problem was first documented in
Slicer/SlicerExecutionModel#137

Modules/ThirdParty/Expat/src/expat/CMakeLists.txt

@@ -196,6 +196,7 @@ INSTALL(FILES
   ${ITK3P_EXPAT_BINARY_DIR}/expatDllConfig.h
   ${ITK3P_EXPAT_SOURCE_DIR}/expat.h
   ${ITK3P_EXPAT_BINARY_DIR}/expat_config.h
+  ${ITK3P_EXPAT_SOURCE_DIR}/expat_external.h
   ${ITK3P_EXPAT_SOURCE_DIR}/itk_expat_mangle.h
   DESTINATION ${ITK3P_INSTALL_INCLUDE_DIR} # TODO: itk_expat.h #include "itkexpat/expat.h"
   COMPONENT Development)