ixgbe: remove circular dependency in ixgbe_mbx.h

DPDK commit message

net/ixgbe/base: remove circular header dependency
Including one header file in second header file should be avoided, so
fix it by forward declaring the struct instead.

Signed-off-by: Barbara Skobiej <barbara.skobiej@intel.com>

Obtained from: DPDK (0bc2af5)
MFC after: 1 week