debian/control: Add itstool build dependency.

Author: mtwebster

debian/control

@@ -8,6 +8,7 @@ Build-Depends:
  gobject-introspection,
  gtk-doc-tools (>= 1.4),
  intltool (>= 0.40.1),
+ itstool,
  libatk1.0-dev (>= 1.32.0),
  libcinnamon-desktop-dev (>= 3.6),
  libexempi-dev (>= 2.2.0),