net: ena: fix compilation error in xtensa architecture

BugLink: http://bugs.launchpad.net/bugs/1798182

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: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 00f17a82 net-next)
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/net/ethernet/amazon/ena/ena_com.h

@@ -38,6 +38,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/gfp.h>
 #include <linux/io.h>
+#include <linux/prefetch.h>
 #include <linux/sched.h>
 #include <linux/sizes.h>
 #include <linux/spinlock.h>