libbpf-tools: Make custom libbpf callback function static

libbpf-tools: Make custom libbpf callback function static

Update all tools to make libbpf_print_fn function static. While
at it, also keep the function signature in a single line since
they fit in a 100-character line.

Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>