meson: ensure correct build order of VAPI

Without this commit spice-client-gtk-3.0.vapi may be built
before spice-client-glib-2.0.vapi if build_jobs > 1. This causes
the build to fail because the former depends on the latter
5 jobs for fix-vapi-build in 14 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #85293
makecheck

00:13:59

failed #85294
makecheck-meson

00:02:35

failed #85295
makecheck_simple

00:03:23

failed #85296
makecheck_simple-meson

00:02:49

failed #85297
make-win

00:02:53

 
Name Stage Failure
failed
makecheck Test
  xz-devel-5.2.4-3.fc29.x86_64                                                  
zip-3.0-23.fc29.x86_64

Complete!
$ git clone ${CI_REPOSITORY_URL/spice-gtk/spice-protocol}
Cloning into 'spice-protocol'...
remote: The project you were looking for could not be found.
fatal: repository 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.freedesktop.org/Cogitri/spice-protocol.git/' not found
ERROR: Job failed: exit code 1
failed
makecheck_simple Test
  xz-devel-5.2.4-3.fc29.x86_64                                                  
zip-3.0-23.fc29.x86_64

Complete!
$ git clone ${CI_REPOSITORY_URL/spice-gtk/spice-protocol}
Cloning into 'spice-protocol'...
remote: The project you were looking for could not be found.
fatal: repository 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.freedesktop.org/Cogitri/spice-protocol.git/' not found
ERROR: Job failed: exit code 1
failed
make-win Test
  xz-devel-5.2.4-3.fc29.x86_64                                                  
zip-3.0-23.fc29.x86_64

Complete!
$ git clone ${CI_REPOSITORY_URL/spice-gtk/spice-protocol}
Cloning into 'spice-protocol'...
remote: The project you were looking for could not be found.
fatal: repository 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.freedesktop.org/Cogitri/spice-protocol.git/' not found
ERROR: Job failed: exit code 1
failed
makecheck_simple-meson Test
  xz-devel-5.2.4-3.fc29.x86_64                                                  
zip-3.0-23.fc29.x86_64

Complete!
$ git clone ${CI_REPOSITORY_URL/spice-gtk/spice-protocol}
Cloning into 'spice-protocol'...
remote: The project you were looking for could not be found.
fatal: repository 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.freedesktop.org/Cogitri/spice-protocol.git/' not found
ERROR: Job failed: exit code 1
failed
makecheck-meson Test
  xz-devel-5.2.4-3.fc29.x86_64                                                  
zip-3.0-23.fc29.x86_64

Complete!
$ git clone ${CI_REPOSITORY_URL/spice-gtk/spice-protocol}
Cloning into 'spice-protocol'...
remote: The project you were looking for could not be found.
fatal: repository 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.freedesktop.org/Cogitri/spice-protocol.git/' not found
ERROR: Job failed: exit code 1