linux/prefetch.h is never explicitly included in ena_com, although
functions from it, such as prefetchw(), are used throughout ena_com.
This is an inclusion bug, and we fix it here by explicitly including
linux/prefetch.h. The bug was exposed when the driver was compiled
for the xtensa architecture.

Fixes: 689b2bda ("net: ena: add functions for handling Low Latency Queues in ena_com")
Fixes: 8c590f97 ("ena: Fix Kconfig dependency on X86")
Signed-off-by: NArthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>