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: ba95c74524 ("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>

tools/bpf/bpftool/main.c

@@ -139,7 +139,7 @@ int detect_common_prefix(const char *arg, ...)
 	strncat(msg, "'", sizeof(msg) - strlen(msg) - 1);
 
 	if (count >= 2) {
-		p_err(msg);
+		p_err("%s", msg);
 		return -1;
 	}