net: qlcnic: include irq.h for irq definitions

The qlcnic driver fails to build on ARM with errors like:

In file included from drivers/net/ethernet/qlogic/qlcnic/qlcnic.h:36:0,
                 from drivers/net/ethernet/qlogic/qlcnic/qlcnic_hw.c:8:
drivers/net/ethernet/qlogic/qlcnic/qlcnic_83xx_hw.h:585:1: error: unknown type name 'irqreturn_t'
 irqreturn_t qlcnic_83xx_clear_legacy_intr(struct qlcnic_adapter *);
 ^

Nothing in the driver is explicitly including the irq definitions, so we
add an include of linux/irq.h to pick them up.

Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: David S. Miller <davem@davemloft.net>