ProjectExplorer: Re-add SUBPROJECT_OPEN_LOCATIONS_CONTEXT_MENU

It was only accidentally unused.

Change-Id: I41702ae2840260af1c131c7454e00ea5e1abee57
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/projectexplorer/projectexplorer.cpp

@@ -238,6 +238,7 @@ const char G_BUILD_CANCEL[]       = "ProjectExplorer.Group.BuildCancel";
 const char RUNMENUCONTEXTMENU[]   = "Project.RunMenu";
 const char FOLDER_OPEN_LOCATIONS_CONTEXT_MENU[]  = "Project.F.OpenLocation.CtxMenu";
 const char PROJECT_OPEN_LOCATIONS_CONTEXT_MENU[]  = "Project.P.OpenLocation.CtxMenu";
+const char SUBPROJECT_OPEN_LOCATIONS_CONTEXT_MENU[]  = "Project.S.OpenLocation.CtxMenu";
 
 } // namespace Constants