Add empty Scottish Gaelic translation files for GParted help manual

GunChleoc has brought to my attention an inability to create the
Scottish Gaelic (gd) translation files for the GParted help
manual using Damned Lies.  In an effort to work around this challenge
I am adding "gd" to LINGUAS in help/Makefile.am, and also adding a
template help/gd/gd.po file for translation.