Don't depend on props.hxx

There is no reference to anything defined in props.hxx, so remove
the dependency here.

simgear/structure/subsystem_mgr.hxx

@@ -29,8 +29,8 @@
 #include <map>
 #include <vector>
 
-#include <simgear/props/props.hxx>
 #include <simgear/timing/timestamp.hxx>
+#include <simgear/structure/SGSharedPtr.hxx>
 #include "SGSmplstat.hxx"