KPTI: Fix build when ftrace enabled

This adds a missing include for when function tracing is enabled.

Signed-off-by: Kees Cook <keescook@chromium.org>
diff --git a/arch/x86/mm/kaiser.c b/arch/x86/mm/kaiser.c
index 28e85ec..526288f 100644
--- a/arch/x86/mm/kaiser.c
+++ b/arch/x86/mm/kaiser.c
@@ -5,6 +5,7 @@
 #include <linux/types.h>
 #include <linux/bug.h>
 #include <linux/init.h>
+#include <linux/ftrace.h>
 #include <linux/interrupt.h>
 #include <linux/spinlock.h>
 #include <linux/mm.h>