Reset main_module_tool_deps in QtPostProcess

The variable was not reset when interating through all the Qt targets,
which resulted in QtTest wronlgy depending on QtWidgetTools.

Task-number: QTBUG-75875
Change-Id: I24da495fa53e9163992a1ed53f5cd7e1c6da5e51
Reviewed-by: Qt CMake Build Bot
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

cmake/QtPostProcess.cmake

@@ -25,6 +25,7 @@ function(qt_internal_create_depends_files)
         set(third_party_deps_seen "")
         set(tool_deps "")
         set(tool_deps_seen "")
+        set(main_module_tool_deps "")
 
         foreach (dep ${depends})
             # Normalize module by stripping leading "Qt::" and trailing "Private"