Add the custom build step for PCH generated through source

This fixes a regression introduced by
04d3a89e

 as it left out the custom build
step for the source code file generated for PCH.

Task-number: QTBUG-42596
Change-Id: I53d5a36b842dcffbde2657910e6a96dca0e99c7b
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>