log: flush log lines, to cope with non-terminal output (pipes)

Although log messages are terminated by a line feed, output still might
get buffered when the program does not talk to a terminal (that it is
aware of). Add an fflush(3) call to the printf(3) sequence such that log
output will immediately show up when writing to pipes or sockets, too.

src/log.c

@@ -204,6 +204,7 @@ static int sr_logv(void *cb_data, int loglevel, const char *format, va_list args
 	}
 
 	g_fprintf(stderr, "%s\n", output);
+	fflush(stderr);
 	g_free(raw_output);
 	g_free(output);