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>

diff --git a/ccan/strmap/LICENSE b/ccan/strmap/LICENSE
new file mode 120000 (symlink)
index 0000000..b7951da
--- /dev/null
+++ b/ccan/strmap/LICENSE
@@ -0,0 +1 @@
+../../licenses/CC0
\ No newline at end of file