hw/9pfs: handle undefined FS_IOC_GETVERSION case in handle_ioc_getversion()

All get_st_gen() implementations except handle_ioc_getversion() have
guard for undefined FS_IOC_GETVERSION. Let's add it there too.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
