MAINTAINERS: Add thunderbolt.h to the Thunderbolt driver entry

The new API header (include/linux/thunderbolt.h) is maintained by the
Thunderbolt driver maintainers.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Michael Jamet <michael.jamet@intel.com>
Reviewed-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>