net: enic: include irq.h for irqreturn_t definitions

The enic driver fails to build on ARM with:

In file included from drivers/net/ethernet/cisco/enic/enic_res.c:40:0:
drivers/net/ethernet/cisco/enic/enic.h:48:2: error: expected specifier-qualifier-list before 'irqreturn_t'
  irqreturn_t (*isr)(int, void *);
  ^

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>