flush stderr after a log message appears

We had the problem that the log file appeared empty for a long time due
to file buffers. Thus we call fflush() after every log message so the
log file gets written out to disk quickly

log.c

@@ -58,6 +58,8 @@ void stderr_log_with_level(const char *level_name, int level, const char *fmt, .
 		vfprintf(stderr, fmt1, ap);
 
 		va_end(ap);
+
+		fflush(stderr);
 	}
 }