PaperSize: Correct the PaperSizeTraits pre-declaration.

* gtk/src/papersize.hg: PaperSizeTraits is a struct, not a class.
clang++ noticed this while building Glom, though I have not run it on all of
gtkmm.
Merge branch 'gtkmm-2-24' of git://git.gnome.org/gtkmm into v2.24.00
JOHNE - Preliminary stuff for generating pkg-config files. Will probably get supplemented depending upon feedback from Todd