tools/build/make.py: Minor typo fix

If the object directory prefix does not exist, make.py previously exited
with a message indicating that the chosen prefix does not exit.

Reviewed by: emaste
Pull request: https://github.com/freebsd/freebsd-src/pull/1348

(cherry picked from commit d55de30efd09ee011bf4604be1c15ce4155775c1)