ixgbe: fix namespace issue with ixgbe_dcb_txq_to_tc

We didn't need the prototype and it was causing namespace complaints so
I made it static.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>