CMake: Install QtHeadersClean.cmake

For prefix builds we need to install the file.
Amends db21bad936.

Change-Id: Ia8e859dc048cc4bd74eb95ed0d2adce9c6b11902
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

cmake/QtBaseGlobalTargets.cmake

@@ -374,6 +374,7 @@ qt_copy_or_install(FILES
                    cmake/QtGenerateLibHelpers.cmake
                    cmake/QtGenerateLibPri.cmake
                    cmake/QtGlobalStateHelpers.cmake
+                   cmake/QtHeadersClean.cmake
                    cmake/QtInstallHelpers.cmake
                    cmake/QtLalrHelpers.cmake
                    cmake/QtModuleConfig.cmake.in