KVM: arm64: Add missing #include of <linux/string.h> in guest.c

arch/arm64/kvm/guest.c uses the string functions, but the
corresponding header is not included.

We seem to get away with this for now, but for completeness this
patch adds the #include, in preparation for adding yet more
memset() calls.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/guest.c

@@ -23,6 +23,7 @@
 #include <linux/err.h>
 #include <linux/kvm_host.h>
 #include <linux/module.h>
+#include <linux/string.h>
 #include <linux/vmalloc.h>
 #include <linux/fs.h>
 #include <kvm/arm_psci.h>