it fails when using make.
make -C /lib/modules/`uname -r`/build M=$PWD modules
make[1]: Entering directory '/usr/src/linux-headers-4.14.3-custom-100hz'
CC [M] /home/poc/poc.o
/home/poc/poc.c: In function ‘init_module’:
/home/poc/poc.c:213:2: error: implicit declaration of function ‘nf_register_hook’ [-Werror=implicit-function-declaration]
nf_register_hook(&pre_hook);
^~~~~~~~~~~~~~~~
/home/poc/poc.c: In function ‘cleanup_module’:
/home/poc/poc.c:228:2: error: implicit declaration of function ‘nf_unregister_hook’ [-Werror=implicit-function-declaration]
nf_unregister_hook(&pre_hook);
^~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
scripts/Makefile.build
20: recipe for target '/home/poc/poc.o' failed
make[2]: *** [/home/poc/poc.o] Error 1
Makefile:1503: recipe for target '_module_/home/poc' failed
make[1]: *** [_module_/home/poc] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-4.14.3-custom-100hz'
Makefile:8: recipe for target 'default' failed
make: *** [default] Error 2
root:/home/poc# error: implicit declaration of function ‘nf_unregister_hook’