tools: bpftool: add bash completion for `bpftool prog load`

Add bash completion for bpftool command `prog load`. Completion for this
command is easy, as it only takes existing file paths as arguments.

Fixes: 49a086c2

 ("bpftool: implement prog load command")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>