sound: oss/vwsnd: Always define vwsnd_mutex

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>