src/meson.build: fix parallel build for libcommon
The build was sometimes failing because config.h is needed before it is built, especially in the gnome-build-meta aarch64 runner which uses -j96. See for example https://gitlab.gnome.org/GNOME/gnome-build-meta/-/jobs/83117
Fixes #237 (closed)
Edited by Abderrahim Kitouni