platform/x86: dell-wmi: Add an event created by Dell Latitude 5495

The Dell Latitude 5495 has the mic mute key.

Signed-off-by: Shih-Yuan Lee (FourDollars) <sylee@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>