tests: change license of arequal-checksum.c to include GPL v2.

In line with the licensing scheme of other files in the project.

Change-Id: Ic4c7dfcd2f2ceaeda9ec05bd37de8ef49a1c6206
BUG: 1168207
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/9203
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>