Fix compilation failure in remote.c

Fix compilation failure in remote.c

A recent patch introduced a few of these:

/home/emaisin/src/binutils-gdb/gdb/remote.c:12862:19: error: format not a string literal and no format arguments [-Werror=format-security]
     error (err_msg);
                   ^

Fix them by replacing the call to error with

  error ("%s", err_msg);

gdb/ChangeLog:

	* remote.c (remote_target::download_tracepoint): Fix format
	string errors.