tools: bpftool: remove spurious line break from error message

The end-of-line character inside the string would break JSON compliance.
Remove it, `p_err()` already adds a '\n' character for plain output
anyway.

Fixes: 9a5ab8bf ("tools: bpftool: turn err() and info() macros into functions")
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>