asearch: Correct license tag to LGPL 2.1+

The _info file for asearch currently gives the license as just "LGPL".
That gives a warning from ccanlint, because it interprets that as the
latest LGPL 3 whereas asearch.h and the license link indicate it's actually
LGPL 2.1+.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/asearch/_info

@@ -10,7 +10,7 @@
  *
  * This version uses macros to be typesafe on platforms which support it.
  *
- * License: LGPL
+ * License: LGPL (v2.1 or any later version)
  * Author: Rusty Russell <rusty@rustcorp.com.au>
  *
  * Example: