strmap: Set the license for strmap to CC0

This matches the license in _info and the source. It also means every
module now has a LICENSE file and so anyone happening to rewrite the
ccan makefiles can use wildcard to locate modules rather than listing
them by hand.
Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>

ccan/strmap/LICENSE

+../../licenses/CC0
\ No newline at end of file