Put proper GPL declarations on the source files

Put proper GPL declarations on the source files

This is GPL best-practice. I have assumed that this package is
intentionally placed under the GPL version 2 only (like Linux or
ConnMan), and not dual- or multiple-licensed under the GPL version 2
"or any later version" (like BlueZ).

As a result, the wording used is similar to what is recommended in
the GPL v2, but modified to omit the "or later" clause.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>