When trying to build the latest BPF selftests, with a debug kernel
config, Pahole 1.30 and CLang 20.1.8 (and GCC 15.2), I got these errors:

  progs/dynptr_success.c:579:9: error: call to undeclared function 'bpf_dynptr_slice'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    579 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 1);
        |                ^
  progs/dynptr_success.c:579:9: note: did you mean 'bpf_dynptr_size'?
  .virtme/build-debug-btf//tools/include/vmlinux.h:120280:14: note: 'bpf_dynptr_size' declared here
   120280 | extern __u32 bpf_dynptr_size(const struct bpf_dynptr *p) __weak __ksym;
          |              ^
  progs/dynptr_success.c:579:7: error: incompatible integer to pointer conversion assigning to '__u64 *' (aka 'unsigned long long *') from 'int' [-Wint-conversion]
    579 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 1);
        |              ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  progs/dynptr_success.c:596:9: error: call to undeclared function 'bpf_dynptr_slice'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    596 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 10);
        |                ^
  progs/dynptr_success.c:596:7: error: incompatible integer to pointer conversion assigning to 'char *' from 'int' [-Wint-conversion]
    596 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 10);
        |              ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I don't have these errors without the debug kernel config from
kernel/configs/debug.config. With the debug kernel, bpf_dynptr_slice()
is not declared in vmlinux.h. It is declared there without debug.config.

The fix is similar to what is done in dynptr_fail.c which is also using
bpf_dynptr_slice(): bpf_kfuncs.h is now included.

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>