tools/debugger: fix make distclean

tools/debugger: fix make distclean

"make distclean" will complain that "-c" is no supported flag for make.

Fix that by using "-C".

The error has been present for a long time, but it was uncovered only
recently.

Fixes: 2400a9a365c5619 ("tools/debugger: Allow make to recurse into debugger/")
Fixes: f9c9b127753e9ed ("tools: fix make distclean")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>