Project: Only add qbs file if it exists

Change-Id: I2d49644b63bad9b7ffa7d088f443ed6ec2fc85ed
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

qtcreator.pri

@@ -187,4 +187,5 @@ qt:greaterThan(QT_MAJOR_VERSION, 4) {
     DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x040900
 }
 
-OTHER_FILES += $$replace(_PRO_FILE_, \\.pro$, .qbs)
+QBSFILE = $$replace(_PRO_FILE_, \\.pro$, .qbs)
+exists($$QBSFILE):OTHER_FILES += $$QBSFILE