My policy is that in a .cpp the first include should always be the corresponding .h one. In this way you have an easy way to test if .h files are self-contained, I hate when you add an include, and find out that you have to include more stuff before that or it won't build...
It is not a big deal here, but I try to be coherent with myself :)
So it should be either in the .h file (I really hope it is not necessary there, it can probably be replaced with forward declarations) or in the .cpp file where glibmm/ustring.h is