Disable qtdocgallery in Qt build

qtdocgallery no longer builds on modern toolchains and seems to be
abandoned upstream. It's not a dependency and not popular, so let's
just disable it given the Qt 5.15 branch is closed upstream and
cannot be fixed.
2 jobs for master in 2 minutes and 33 seconds (queued for 127 minutes and 1 second)
latest
Status Job ID Name Coverage
  Test
passed #95759
smoke-test

00:01:50

passed #95758
unit-test

00:00:42