docs: explicitly declare gir build dependencies

As the path to the gir file is passed to hotdoc.generate_doc() and not the build target itself, meson doesn't know about the dependency.

In turn, as the CI doesn't build everything before building the documentation target, some gir files might not exist, for instance in the case of gst-rtsp-server, causing the output documentation to be empty.

The error occurred silently because hotdoc accepts wildcards for *-sources arguments, thus it won't warn about a missing gir file as it is legitimate for glob matching to resolve to nothing.