ustring.h: Comment On Last Commit for Specialization

Mention that the new specialization for Stringify for const char[N] is to
avoid ambiguous constructor overloads for later versions of Visual C++.

	* glib/glibmm/ustring.h: Mention in the comments for the
	  specialization on Stringify for const char[N] is for later
	  versions of Visual C++, to avoid ambiguous contructor overloads.

https://bugzilla.gnome.org/show_bug.cgi?id=736778