ppp: fix compile warning

Signed-off-by: Hauke Mehrtens <[email protected]>

package/network/services/ppp/patches/500-add-pptp-plugin.patch

@@ -1083,6 +1083,7 @@
 +#include <string.h>
 +#include <ctype.h>
 +#include <fcntl.h>
++#include "pppd.h"
 +#include "pptp_msg.h"
 +#include "pptp_ctrl.h"
 +#include "pptp_options.h"