build: meson: fix dependency to gdbus generated headers
libnm has a dependency on 'libnmdbus_dep', which contains 'link_with: libnmdbus'. This however only enforces that libnm is linked after the libnmdbus static library is built; it doesn't give any guarantees about the compilation phase.
We need to make libnm compilation depend on the generated header files. The output of 'gnome.gdbus_codegen' is an array with the header file in the second position; use it to add a proper dependency. Unfortunately this works only with meson >= 0.46.
In the future libnm will no longer use gdbus generated code and this dependency will not be needed anymore.