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>