meson-build: Use relative path in gtk-doc
So meson searches for source files in the right place by itself (source directory or build directory). meson.project_source_root() prevents it from doing so, while meson.current_source_dir() is simply useless.
Fixes: #30 (closed)
Related: xfce4-dev-tools!140 (comment 100394)
Edited by Gaël Bonithon