htable: relicense under LGPL

Various LGPL components depend on it, via ccan/likely.  ccan/likely
really only needs it when CCAN_LIKELY_DEBUG is set, but making it a
conditional dependency is a bit nasty if defining that changes the
license.

So this is the simplest fix.  I might relicense under PD or BSD later,
since the likely module should probably have an even more liberal
license.

ccan/htable/LICENSE

@@ -1 +1 @@
-../../licenses/GPL-2
+../../licenses/LGPL-2.1

ccan/htable/_info

@@ -98,7 +98,7 @@
  *		return 0;
  *	}
  *
- * License: GPL (v2 or any later version)
+ * License: LGPL (v2.1 or any later version)
  * Author: Rusty Russell <rusty@rustcorp.com.au>
  */
 int main(int argc, char *argv[])