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
17 jobs for bg/gtk-doc in 55 minutes and 10 seconds (queued for 23 seconds)
latest
Status Job ID Name Coverage
  Test
passed #568043
allowed to fail
checkpatch

00:01:09

manual #568048
allowed to fail manual
t_centos:7.5.1804
passed #568049
t_centos:7.6.1810

00:24:04

passed #568055
t_debian:10

00:14:34

passed #568054
t_debian:9

00:16:15

passed #568057
t_debian:sid

00:18:55

manual #568056
allowed to fail manual
t_debian:testing
passed #568044
t_fedora:28

00:25:52

passed #568045
t_fedora:29

00:38:12

passed #568046
t_fedora:30

00:26:48

manual #568047
allowed to fail manual
t_fedora:rawhide
passed #568050
t_ubuntu:16.04

00:41:08

passed #568051
t_ubuntu:18.04

00:33:18

manual #568053
allowed to fail manual
t_ubuntu:devel
manual #568052
allowed to fail manual
t_ubuntu:rolling
 
  External
passed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/670/

passed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/670/

05:53:50