Fixed FTBFS in recent distros caused by flex arrays usage in kppp.

Based on the suggestion found here: https://github.com/UAVCAN/libuavcan/issues/116#issuecomment-346879320 (thanks!)

Signed-off-by: Michele Calgaro <michele.calgaro@yahoo.it>
Signed-off-by: Slávek Banko <slavek.banko@axis.cz>

kppp/opener.cpp

@@ -342,7 +342,13 @@ void Opener::mainLoop() {
 //
 int Opener::sendFD(int fd, struct ResponseHeader *response) {
 
-  struct { struct cmsghdr cmsg; int fd; } control;
+  union {
+    struct {
+      char _cmsg[sizeof(struct cmsghdr)];
+      char data[sizeof(int)];  // int fd field
+    };
+    struct cmsghdr cmsg;
+  } control;
   struct msghdr	msg;
   struct iovec iov;
 

kppp/requester.cpp

@@ -80,7 +80,13 @@ Requester::~Requester() {
 // Receive file name and file descriptors from envoy
 //
 int Requester::recvFD() {
-  struct { struct cmsghdr cmsg; int fd; } control;
+  union {
+    struct {
+      char _cmsg[sizeof(struct cmsghdr)];
+      char data[sizeof(int)];  // int fd field
+    };
+    struct cmsghdr cmsg;
+  } control;
   struct msghdr	msg;
   struct ResponseHeader response;