While the conversion of BKL to mutex in commit 645ef9ef, the mutex
definition was put in a wrong place inside #ifdef WSND_DEBUG, which
leads to the build error.  Just move it outside the ifdef.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>