smc91x: Add missing #inclusion of <linux/irq.h>

Add missing #inclusion of <linux/irq.h>.  Without it, the following error can
occur with the irqflags fixup patches applied:

drivers/net/smc91x.c: In function 'smc_probe':
drivers/net/smc91x.c:1987:2: error: implicit declaration of function 'irq_canonicalize'

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>