rtc: lib_test: add MODULE_LICENSE

rtc: lib_test: add MODULE_LICENSE

commit 5546e3dfb65a4389e747766ac455a50c3675fb0f upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>