vcxproj: fix custom build steps in different build variants

There's a comment in VCXProjectWriter::outputFileConfigs that
states: "We need to check if the file has any custom build step.
If there is one then it has to be included with 'CustomBuild
Include'".
This patch adds the code to the comment...

Task-number: QTBUG-30373
Change-Id: Ibfef3c80630e08c743bfadce299a8b6a0c58411f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>