drivers/media/dvb/dvb-core/dvb_frontend.c: needs semaphore.h

Fixes:

  v4l/dvb_frontend.c: In function 'dvb_frontend_stop':
  v4l/dvb_frontend.c:707: error: implicit declaration of function 'init_MUTEX'

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=14609

Reported-by:  <tstrelar@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>