mfd: Add missing mutex.h inclusion to WM8994 core.h

struct wm8994 includes a mutex so we need to include mutex.h before we
declare it. All current users rely on this being done implicitly.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>