libbpf: Refactor btf__add_enum() for future code sharing

Refactor btf__add_enum() function to create a separate
function btf_add_enum_common() so later the common function
can be used to add enum64 btf type. There is no functionality
change for this patch.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062615.3718063-1-yhs@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>