RDMA/i40iw: include linux/irq.h

We get a build failure on ARM unless the header is included explicitly:

drivers/infiniband/hw/i40iw/i40iw_verbs.c: In function 'i40iw_get_vector_affinity':
drivers/infiniband/hw/i40iw/i40iw_verbs.c:2747:9: error: implicit declaration of function 'irq_get_affinity_mask'; did you mean 'irq_create_affinity_masks'? [-Werror=implicit-function-declaration]
  return irq_get_affinity_mask(msix_vec->irq);
         ^~~~~~~~~~~~~~~~~~~~~
         irq_create_affinity_masks
drivers/infiniband/hw/i40iw/i40iw_verbs.c:2747:9: error: returning 'int' from a function with return type 'const struct cpumask *' makes pointer from integer without a cast [-Werror=int-conversion]
  return irq_get_affinity_mask(msix_vec->irq);

Fixes: 7e952b19

 ("i40iw: Implement get_vector_affinity API")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>