kernel/extable.c: remove duplicated include

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/extable.c

@@ -18,7 +18,6 @@
 #include <linux/ftrace.h>
 #include <linux/memory.h>
 #include <linux/module.h>
-#include <linux/ftrace.h>
 #include <linux/mutex.h>
 #include <linux/init.h>