libbpf-tools: fix an error message

A wrong argument (-errno) was passed to strerror, fix it.

Signed-off-by: Anton Protopopov <[email protected]>

libbpf-tools/syscount.c

@@ -434,7 +434,7 @@ int main(int argc, char **argv)
  }
 
  if (signal(SIGINT, sig_int) == SIG_ERR) {
- warn("can't set signal handler: %s\n", strerror(-errno));
+ warn("can't set signal handler: %s\n", strerror(errno));
  goto cleanup_obj;
  }