Fix building ixgbe with gcc, it doesn't like nested extern declarations.

The fix is to move the extern declaration ix_crcstrip out of
ixgbe_setup_hw_rsc.

sys/dev/ixgbe/ix_txrx.c

@@ -50,6 +50,8 @@
 #include <net/netmap.h>
 #include <sys/selinfo.h>
 #include <dev/netmap/netmap_kern.h>
+
+extern int ix_crcstrip;
 #endif
 
 /*
@@ -1240,7 +1242,6 @@ ixgbe_setup_hw_rsc(struct rx_ring *rxr)
 	rdrxctl = IXGBE_READ_REG(hw, IXGBE_RDRXCTL);
 	rdrxctl &= ~IXGBE_RDRXCTL_RSCFRSTSIZE;
 #ifdef DEV_NETMAP /* crcstrip is optional in netmap */
-	extern int ix_crcstrip;
 	if (adapter->ifp->if_capenable & IFCAP_NETMAP && !ix_crcstrip)
 #endif /* DEV_NETMAP */
 	rdrxctl |= IXGBE_RDRXCTL_CRCSTRIP;