audiod: Fix time diff warning log message.

audiod: Fix time diff warning log message.

The format string contains %lu to print a long unsigned, but we
possibly multiply the value with -1, which can lead to output like

	Jun 13 13:36:37 baader (3) compute_time_diff: time diff jump: 4294808018ms

Fix this by replacing the multiplication with an additional format
string directive to print the leading +/- explicitly. This is easy
since we already have the sign in a variable, and it avoids any
integer conversion/overflow issues.