tools: bpftool: fix format string for p_err() in detect_common_prefix()

There is one call to the p_err() function in detect_common_prefix()
where the message to print is passed directly as the first argument,
without using a format string. This is harmless, but may trigger
warnings if the "__printf()" attribute is used correctly for the p_err()
function. Let's fix it by using a "%s" format string.

Fixes: ba95c745 ("tools: bpftool: add "prog run" subcommand to test-run programs")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>