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>

net/ipv4/fib_notifier.c

@@ -6,7 +6,6 @@
 #include <linux/export.h>
 #include <net/net_namespace.h>
 #include <net/fib_notifier.h>
-#include <net/netns/ipv4.h>
 #include <net/ip_fib.h>
 
 int call_fib4_notifier(struct notifier_block *nb,