[source] ppp: fix compile warning

[LEDE Commits]

jow pushed a commit to source.git, branch lede-17.01:
https://git.lede-project.org/d63eb474b3fc6cf5fb57dbe985689df3f7cf0b0e

commit d63eb474b3fc6cf5fb57dbe985689df3f7cf0b0e
Author: Hauke Mehrtens <hauke at hauke-m.de>
AuthorDate: Sun Oct 15 14:19:20 2017 +0200

    ppp: fix compile warning
    
    Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
    (cherry picked from commit f40fd43ab2fe0f5c707a07eb036e84af086f3066)
---
 package/network/services/ppp/patches/500-add-pptp-plugin.patch | 1 +
 1 file changed, 1 insertion(+)

diff --git a/package/network/services/ppp/patches/500-add-pptp-plugin.patch b/package/network/services/ppp/patches/500-add-pptp-plugin.patch
index d984e1b..54a160d 100644
--- a/package/network/services/ppp/patches/500-add-pptp-plugin.patch
+++ b/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"