libnm: avoid deprecated warning for NMVpnPluginInfo

When merely including "nm-vpn-plugin-info.h" (or "NetworkManager.h")
gcc raises warnings like:
  "Not available before 1.2 [-Werror=deprecated-declarations]"

The problem is that the NMVpnPluginInfo typedef itself is marked as
deprecated but also used by other functions like nm_vpn_plugin_info_get_name().

    typedef struct {
        int field;
    } Foo G_UNAVAILABLE(1,2);

    G_UNAVAILABLE(1,2)
    void deprecated_function (Foo *foo);

    warning: ‘Foo’ is deprecated: Not available before 1.2 [-Wdeprecated-declarations]

I think that when a function is itself deprecated, gcc should not warn about
the use of a deprecated typedef.
Gcc's documentation states: "Note that the warnings only occur for
uses and then only if the type is being applied to an identifier
that itself is not being declared as deprecated.".
Apparently, this only works for structs, but not for typedef of structs.

Anyway. Remove the deprecation from NMVpnPluginInfo to avoid the compiler
warning.

https://bugzilla.gnome.org/show_bug.cgi?id=753098

Fixes: d6226bd987

libnm-core/nm-vpn-plugin-info.h

@@ -47,7 +47,7 @@ G_BEGIN_DECLS
 typedef struct {
 	NM_AVAILABLE_IN_1_2
 	GObject parent;
-} NMVpnPluginInfo NM_AVAILABLE_IN_1_2;
+} NMVpnPluginInfo;
 
 typedef struct {
 	NM_AVAILABLE_IN_1_2