drivers: ivshmem: msi.h header only required for ivshmem-doorbell

Only include the MSI header when ivshmem-doorbell is employed, as the
ivshmem data structures already use the same protection for msi-related
structures.

Signed-off-by: Rodrigo Cataldo <rodrigo.cataldo@huawei.com>
Co-authored-by: Henri Xavier <datacomos@huawei.com>