md4: fix license

As ccanlint now says:
  Source files don't contain incompatible licenses (license_file_compat): FAIL
  /home/rusty/devel/cvs/ccan/ccan/md4/md4.c:Found boilerplate for license 'GPLv2+' which is incompatible with 'LGPLv2+'

This is actually GPL code!

Add LICENSE link, too.

ccan/md4/LICENSE

+../../licenses/GPL-2
\ No newline at end of file

ccan/md4/_info

@@ -31,7 +31,7 @@
  *		return 0;
  *	}
  *
- * License: LGPL (v2.1 or any later version)
+ * License: GPL (v2 or any later version)
  */
 int main(int argc, char *argv[])
 {