rtc: lib_test: add MODULE_LICENSE

As the documentation states, "The exact license information can only be
determined via the license information in the corresponding source files."
and the SPDX identifier has the proper information.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210810212008.631359-1-alexandre.belloni@bootlin.com