BUG: Include find GTest CMake code when installed

Even if GTest is not an exported public or private dependency it may
still be used by application if all the ITK modules are selected for
use.

Author: blowekamp

Modules/ThirdParty/GoogleTest/CMakeLists.txt

@@ -10,6 +10,9 @@ if(NOT ITK_BINARY_DIR)
   set(GTEST_ROOT \"${GTEST_ROOT}\")
   find_package(GTest REQUIRED)
 endif()
+")
+    set(ITKGoogleTest_EXPORT_CODE_INSTALL "
+  find_package(GTest REQUIRED)
 ")
   endif()
 endif ()