qbs build: Fix warnings on Windows

Change-Id: Ic62200d30be054128fd810ae85e7b8eb1900be89
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

qbs/imports/QtcPlugin.qbs

@@ -25,7 +25,10 @@ QtcProduct {
         name: "Qt.test"
     }
 
-    cpp.internalVersion: ""
+    Properties {
+        condition: qbs.targetOS.contains("unix")
+        cpp.internalVersion: ""
+    }
     cpp.defines: base.concat([name.toUpperCase() + "_LIBRARY"])
     cpp.sonamePrefix: qbs.targetOS.contains("macos")
         ? "@rpath"