watchedpath: Flush stdio more often

When stdout/stderr is redirected to a file, they are typically opened in
fully-buffered mode. This makes it difficult to watch logs in real time,
unless the buffer is flushed every time a new message is written.

src/watchedpath.vala

@@ -43,6 +43,7 @@ namespace onchanged {
             if (_monitors.get(path) == null) {
                 if (onchanged.debug) {
                     stdout.printf("Watching path %s\n", path);
+                    stdout.flush();
                 }
                 FileMonitor mon;
                 try {
@@ -51,6 +52,7 @@ namespace onchanged {
                     stderr.printf(
                         "Failed to monitor %s: %s\n", path, e.message
                     );
+                    stderr.flush();
                     return;
                 }
                 mon.changed.connect(handle_event);
@@ -122,6 +124,7 @@ namespace onchanged {
                 .replace("$%", event);
             if (onchanged.debug) {
                 stdout.printf("%s\n", command);
+                stdout.flush();
             }
             try {
                 string[] argv;