MSVC_NMake/generate-msvc.mak: Fix generating gtk/gtkmm/wrap_init.cc
MSVC_NMake/generate-msvc.mak: Fix generating gtk/gtkmm/wrap_init.cc

The list of .hg headers went a bit too long for the for loop to generate
the .bat file that is in turn used to generate the wrap_init.cc source
file, so just drop the '.hg' filename extension when looping through the
.hg files while generating the .bat file, but append the '.hg' filename
extension when writing the final temporary .bat file.  Sigh...

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