pppd: make pid directory before create the pid file

If multilink feature is not enabled, the '/var/run/pppd' directory
won't be created before adding pid file.

Fixes error message:
'Failed to create pid file /var/run/pppd/pppoe-wan.pid: No such file or directory'

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>

pppd/main.c

@@ -923,6 +923,9 @@ create_pidfile(int pid)
 {
     FILE *pidfile;
 
+#ifndef PPP_WITH_TDB
+    mkdir_recursive(PPP_PATH_VARRUN);
+#endif
     slprintf(pidfilename, sizeof(pidfilename), "%s/%s.pid",
 	     PPP_PATH_VARRUN, ifname);
     if ((pidfile = fopen(pidfilename, "w")) != NULL) {