libnm-core: add NMSettingIPTunnel

Add a generic NMSettingTunnel which describes properties of tunnels
over IPv4 and IPv6 (GRE, SIT, IPIP, IPIP6, IP6GRE, etc.). Since those
tunnel types all have similar properties it makes sense to have a
single setting type for them.

libnm/NetworkManager.h

@@ -61,6 +61,7 @@
 #include <nm-setting-gsm.h>
 #include <nm-setting-infiniband.h>
 #include <nm-setting-ip-config.h>
+#include <nm-setting-ip-tunnel.h>
 #include <nm-setting-ip4-config.h>
 #include <nm-setting-ip6-config.h>
 #include <nm-setting-olpc-mesh.h>