net/ipv4/udp_tunnel_core.c: remove superfluous header files from udp_tunnel_core.c

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

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp_tunnel_core.c

@@ -2,11 +2,8 @@
 #include <linux/module.h>
 #include <linux/errno.h>
 #include <linux/socket.h>
-#include <linux/udp.h>
-#include <linux/types.h>
 #include <linux/kernel.h>
 #include <net/dst_metadata.h>
-#include <net/net_namespace.h>
 #include <net/udp.h>
 #include <net/udp_tunnel.h>