libbpf: Improve error logging for mismatched BTF kind cases

Instead of printing out integer value of BTF kind, print out a string
representation of a kind.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200818223921.2911963-2-andriin@fb.com