meson: fix disabling introspection

Even with build_by_default: false meson seems to probe for
g-ir-scanner.
5 jobs for fix-cross-introspection in 6 minutes and 55 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
failed #85298
makecheck

00:03:31

failed #85299
makecheck-meson

00:03:32

failed #85300
makecheck_simple

00:04:14

failed #85301
makecheck_simple-meson

00:06:54

failed #85302
make-win

00:02:23

 
Name Stage Failure
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_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
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
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
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