Update dovecot2-pigeonhole to 0.4.6.

Version 0.4.5 introduced one new 'feature', but it caused build
problems. This release fixes that. If you've successfully installed
0.4.5, you do not need to upgrade.

Changelog v0.4.6:

- After make distclean, the distributed tarball would fail to recompile.
  This causes problems for some distribution builds.

(taca)