This is a very simple solution to add the main SPDX based expressions
to the commit hook. For the main SPDX expressions, they specific
strings are detected, yet it does not really handle the full scope of
expressions.
The scope of this commit only is to avoid that any properly licensed file
is prevented from being pushed to our repositories, when a commonly
used SPDX expression is used. A generic parser for SPDX expressions
will be the better solution.