package/mender: set version in go linker

This is used when calling the mender client with the
-version option and it says "unknown" if not set in
linker.

Now it displays the following:

    # mender -version
    1.4.0
    runtime: go1.10.2

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>