lib: linear_ranges: Add missing MODULE_LICENSE()

When linear_ranges is compiled as module we get warning
about missing MODULE_LICENSE(). Fix it by adding
MODULE_LICENSE("GPL") as is suggested by SPDX and EXPORTs.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20200509151519.GA7100@localhost.localdomain


Signed-off-by: Mark Brown <broonie@kernel.org>