selftests/bpf: Make bpf_dynptr_is_rdonly() prototyype consistent with kernel

Currently kernel kfunc bpf_dynptr_is_rdonly() has prototype ...

  __bpf_kfunc bool bpf_dynptr_is_rdonly(struct bpf_dynptr_kern *ptr)

... while selftests bpf_kfuncs.h has:

  extern int bpf_dynptr_is_rdonly(const struct bpf_dynptr *ptr) __ksym;

Such a mismatch might cause problems although currently it is okay in
selftests. Fix it to prevent future potential surprise.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230517040409.4024618-1-yhs@fb.com