stdenv/setup.sh: Always run postPatch hook.

I'm not sure whether this was by intention (was it, @edolstra? then we might need to fix the documentation), but so far `postPatch` hooks were silently skipped whenever the `patches` list was empty. This change could possibly change the build results of the following packages:

 * gcc
 * cmake (264)
 * systemtap
 * quemu-kvm

These packages all have in common that they have a `postPatch` hook and the `patches` list can be empty when certain conditions are met.