igb: gcc-3.4.6 fix

forward declaration of inline function should be avoided, or
old gcc cannot compile.

Reported-by: Teck Choon Giam <giamteckchoon@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>