microblaze: Add missing empty ftrace.h file

Signed-off-by: Michal Simek <monstr@monstr.eu>
diff --git a/arch/microblaze/include/asm/ftrace.h b/arch/microblaze/include/asm/ftrace.h
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/arch/microblaze/include/asm/ftrace.h
@@ -0,0 +1 @@
+