waf: upgrade to 2.0.21

waf: upgrade to 2.0.21

This commit message was wrong:

    commit 5fc3a71d0f54b176d3cb2e399718d0468507e797
    Author: David Mulder <dmulder@suse.com>
    Date:   Mon Aug 24 13:12:46 2020 -0600

        waf: upgrade to 2.0.20

        This contain an important change:
        "Fix gccdeps.scan() returning nodes that no longer exist on disk."
        https://gitlab.com/ita1024/waf/-/merge_requests/2293

        Signed-off-by: David Mulder <dmulder@suse.com>
        Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
The fix was in in waf master, but not included in 2.0.20,
but it's now included in 2.0.21.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>