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