[network-manager-sstp] Fix a problem when compiling against pppd 2.4.7.

[Eivind Næss <eivnaes src gnome org>]

commit f885bda23858f565ea3171c684031a7ccd4418e4
Author: Eivind Næss <eivnaes yahoo com>
Date:   Tue Jun 8 14:03:21 2021 -0700

    Fix a problem when compiling against pppd 2.4.7.
    
    The function mppe_keys_isset() doesn't exist and will be provided by
    nm-sstp-pppd-mppe.* files. Also, pppd won't provide mppe.h with this
    version.

 src/nm-sstp-pppd-plugin.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
---
diff --git a/src/nm-sstp-pppd-plugin.c b/src/nm-sstp-pppd-plugin.c
index 5211acd..abe4d09 100644
--- a/src/nm-sstp-pppd-plugin.c
+++ b/src/nm-sstp-pppd-plugin.c
@@ -30,7 +30,6 @@
 #include <pppd/ipv6cp.h>
 #include <pppd/chap-new.h>
 #include <pppd/chap_ms.h>
-#include <pppd/mppe.h>
 #include <pppd/eap.h>
 
 #include "nm-default.h"
@@ -787,7 +786,7 @@ nm_snoop_recv(unsigned char *buf, int len)
     }
 
     /* Don't bother if the keys aren't set yet */
-    if (!mppe_keys_set) {
+    if (!mppe_keys_isset()) {
         return;
     }