CONTRIBUTING: drop blktap2 and add tools/libs

Blktap2 is gone and tools/libs is missing in the document.

Signed-off-by: Wei Liu <wl@xen.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

CONTRIBUTING

@@ -13,7 +13,7 @@ Most of the Xen Project code is licensed under GPLv2, but a number of
 directories are primarily licensed under different licenses.
 
 Most notably:
- - tools/blktap2      : BSD-Modified
+ - tools/libs         : LGPL v2.1
  - tools/libxc        : LGPL v2.1
  - tools/libxl        : LGPL v2.1
  - tools/xl           : LGPL v2.1