device-nokia-n900: fix missing makedepend
pmbootstrap > 3.3.1 is stricter with the dependencies, it will only install packages that are in makedepends during builds. Before it also installed packages from depends.
Add postmarketos-mvcfg to makedepends, as it gets used in build().
postmarketos-mvcfg will be removed in the future (see #3580), but let's unbreak the build first.
Related: pmbootstrap#2574 (closed)