Remove warnings in Info.plist generation with non-Xcode toolchains.
Remove warnings in Info.plist generation with non-Xcode toolchains.

If using a plain "system" toolchain not based on Xcode, plist generation
will no longer print useless warnings about missing plist files.

Change-Id: I0bf349e4096d7340f2ef91bb449f570591990cdd
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@theqtcompany.com>

gatzka/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs