Commit 48ff7a62 added the QEMU Guest Agent tool with the
optional ".exe" suffix for Windows hosts, but forgot to use
this suffix in the 'clean' rule. Calling this rule let a dangling
executable in the build directory.
Correct this by using the proper optional suffix.

Fixes: 48ff7a62
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20190427161322.24642-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>