libbpf: Make __kptr and __kptr_ref unconditionally use btf_type_tag() attr

It will be annoying and surprising for users of __kptr and __kptr_ref if
libbpf silently ignores them just because Clang used for compilation
didn't support btf_type_tag(). It's much better to get clear compiler
error than debug BPF verifier failures later on.

Fixes: ef89654f ("libbpf: Add kptr type tag macros to bpf_helpers.h")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-3-andrii@kernel.org