dma/idxd: fix build on Windows

dma/idxd: fix build on Windows

Windows compilation gives us a splat:
In file included from ../drivers/dma/idxd/idxd_pci.c:10:
In file included from ..\drivers\dma\idxd/idxd_internal.h:11:
..\drivers\dma\idxd/idxd_hw_defs.h:46:21: error: expected member name or
 ';' after declaration specifiers
        uint16_t __reserved[13];
        ~~~~~~~~           ^
1 error generated.

Ironically, __reserved is probably a reserved token.
Some drivers that build fine on Windows have structs with a "reserved"
field, let's go with this.

Fixes: 82147042d062 ("dma/idxd: add datapath structures")

Signed-off-by: David Marchand <david.marchand@redhat.com>