ustring.h: Add Specialization for Stringify<const char[N]>

This adds a specialization for string literals in the form of const char[N]
as some compilers would take such things to have ambiguous constructor
overloads, such as Visual Studio, which take string literals in this form.

	* glib/glibmm/ustring.h: Add a specialization for Stringify for
	  const char[N] so that ambiguous constructor overloads can be
	  avoided for string literals in this form.

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