A misplaced $(quiet-@) meant that "make clean" and "make distclean" did
not work properly.

Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>