net/ipv4/xfrm4_tunnel.c: remove superfluous header files from xfrm4_t…

…unnel.c

xfrm4_tunnel.c hasn't use any macro or function declared in mutex.h and ip.h
Thus, these files can be removed from xfrm4_tunnel.c safely without affecting
the compilation of the net module.

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_tunnel.c

@@ -8,9 +8,7 @@
 
 #include <linux/skbuff.h>
 #include <linux/module.h>
-#include <linux/mutex.h>
 #include <net/xfrm.h>
-#include <net/ip.h>
 #include <net/protocol.h>
 
 static int ipip_output(struct xfrm_state *x, struct sk_buff *skb)