hwrng: xgene: Add explicit io.h include

Commit 0788257a ("hwrng: Explicitly include correct DT includes")
removed an implicit include of io.h. On most architectures, there's
still an implicit include of it, but not on s390. Enabling COMPILE_TEST
in commit 1ce1cd82 ("hwrng: Enable COMPILE_TEST for more drivers")
exposed this.

Fixes: 0788257a ("hwrng: Explicitly include correct DT includes")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308042049.8R2tNRoo-lkp@intel.com/Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/xgene-rng.c

@@ -14,6 +14,7 @@
 #include <linux/hw_random.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/mod_devicetable.h>
 #include <linux/platform_device.h>