MSVC_NMake/generate-msvc.mak: Fix generating gtkmm.rc
MSVC_NMake/generate-msvc.mak: Fix generating gtkmm.rc

Somehow the rule was accidentally removed, so fix the build by
re-instating it.

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm