doc: fix gtkdoc when orc is used as a subproject

meson.source_root() will point to the root of the top level
project, and should not be used to compute paths in the current
project.
4 jobs for docs-fix-subproject in 1 minute and 49 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #468282
debian sid

00:00:46

passed #468285
docker windows 1607
msys2

00:00:57

passed #468283
docker windows 1607
vs2017 amd64

00:00:58

passed #468284
1607 windows docker
vs2017 x86

00:01:01