libbpf: Forward-declare bpf_stats_type for systems with outdated UAPI headers

Systems that doesn't yet have the very latest linux/bpf.h header, enum
bpf_stats_type will be undefined, causing compilation warnings. Prevents this
by forward-declaring enum.

Fixes: 0bee1067 ("libbpf: Add support for command BPF_ENABLE_STATS")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200621031159.2279101-1-andriin@fb.com