gunit: Adopt SPDX License tag

gunit: Adopt SPDX License tag

Adopt SPDX license tag for all the source files, those already have
LGPL 2.1 boilerplate in them and those missing license information.
All the files in the project fall under project license, hence
explicitly adding LGPL-2.1-only identifier to the files missing license
information. Adopting SPDX license helps the compliance tools to
determine the license and also helps in reducing the repetitive license
boilerplate across source files.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>