For really fast machines, an edge-case may exist where dpv(3) may be
built before contrib dependency, dialog(3). Add dialog(3) to the list
of _prebuild_libs to ensure that this does not happen.

Tested on:	11.0-CURRENT amd64 @ r274205
Thanks to:	kargl, Larry Rosenman <ler@lerctr.org>, ngie, markj
Recommended by:	ngie
Reviewed by:	ngie, markj
MFC after:	21 days
X-MFC-to:	stable/10 stable/9
X-MFC-with:	274116 274120 274121 274123 274144 274146 274192 274203