net: ipv4: remove superfluous header files from fib_notifier.c

fib_notifier.c hasn't use any macro or function declared
in net/netns/ipv4.h.

Thus, these files can be removed from fib_notifier.c safely
without affecting the compilation of the net/ipv4 module.

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Link: https://lore.kernel.org/r/20210928164011.1454-1-liumh1@shanghaitech.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>