Fix build after r291620.

"don't know how to make /Versions.def. Stop"

This was trying to define a target in bsd.symver.mk based on LIBCDIR which was
not yet defined. Switching the order of inclusion of bsd.prog.mk and
bsd.symver.mk fixes it and seems fine.

Pointyhat to: bdrewery
Sponsored by: EMC / Isilon Storage Division