This is necessary to be able to include <linux/msi.h> when
CONFIG_GENERIC_MSI_IRQ_DOMAIN is enabled. Without this, a build with
CONFIG_GENERIC_MSI_IRQ_DOMAIN fails with:

   In file included from drivers//ata/ahci.c:45:0:
>> include/linux/msi.h:226:10: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
             msi_alloc_info_t *arg);
             ^~~~~~~~~~~~~~~~
             sg_alloc_fn
   include/linux/msi.h:230:9: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
            msi_alloc_info_t *arg);
            ^~~~~~~~~~~~~~~~
            sg_alloc_fn
   include/linux/msi.h:239:12: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
               msi_alloc_info_t *arg);
               ^~~~~~~~~~~~~~~~
               sg_alloc_fn
   include/linux/msi.h:240:22: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
     void  (*msi_finish)(msi_alloc_info_t *arg, int retval);
                         ^~~~~~~~~~~~~~~~
                         sg_alloc_fn
   include/linux/msi.h:241:20: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
     void  (*set_desc)(msi_alloc_info_t *arg,
                       ^~~~~~~~~~~~~~~~
                       sg_alloc_fn
   include/linux/msi.h:316:18: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
           int nvec, msi_alloc_info_t *args);
                     ^~~~~~~~~~~~~~~~
                     sg_alloc_fn
   include/linux/msi.h:318:29: error: unknown type name 'msi_alloc_info_t'; did you mean 'sg_alloc_fn'?
            int virq, int nvec, msi_alloc_info_t *args);
                                ^~~~~~~~~~~~~~~~
                                sg_alloc_fn
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>