build: fix errors when building with gtk-doc 1.32

gtkdoc-scan 1.32 performs stricter checks on structures definitions
and so it complains on:

 /build/networkmanager/src/NetworkManager/libnm/./nm-vpn-plugin-old.h:0: warning: partial declaration (struct) : typedef struct {
 	NM_DEPRECATED_IN_1_2
 	GObject parent;
 } NMVpnPluginOld NM_DEPRECATED_IN_1_2;

because of the unrecognized token 'NM_DEPRECATED_IN_1_2'.

Pass all allowed macros to gtkdoc-scan through the --ignore-decorators
argument.

https://gitlab.gnome.org/GNOME/gtk-doc/issues/98
#238