Make org-column-title always fixed pitch

Since this face needs to line up with columns that are in a fixed pitch font face, this one should also always be in fixed pitch.

An example, given an org file with this content:

#+TITLE: Test

* DONE [#A] Something to do                                                             :foo:
* TODO [#B] Another thing to do                                                         :foo:bar:

To see the below effect, enable column view with C-u C-c C-x C-c.

First picture, before this change, see how the headline is in variable pitch, not lining up with the columns:

Variable pitch mode, after this change, column headers now line up with the columns below: