When compiling with -Wstrict-prototypes gcc catches a static
I missed.

./ixgbe_main.c:4254: warning: no previous prototype for 'ixgbe_fwd_ring_down'
Reported-by: NPhillip Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: NJohn Fastabend <john.r.fastabend@intel.com>
Tested-by: NPhil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>