libbpf: Fix another __u64 printf warning

Fix yet another printf warning for %llu specifier on ppc64le. This time size_t
casting won't work, so cast to verbose `unsigned long long`.

Fixes: 166750bc

 ("libbpf: Support libbpf-provided extern variables")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191219052103.3515-1-andriin@fb.com