Build-depend on GLib and pkg-config

We compile some example C code, mainly to prove that we still can.
The build currently fails in apertis:16.12:snapshots because of this
undeclared build-dependency.
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4870

debian/control

@@ -5,6 +5,8 @@ Maintainer: Apertis package maintainers <packagers@lists.apertis.org>
 Build-Depends:
  debhelper (>= 9),
  hotdoc (>= 0.8),
+ libglib2.0-dev,
+ pkg-config,
 Standards-Version: 3.9.8
 
 Package: apertis-designs