Fix qbs build.

Change-Id: I3d335648bed71faf92079b08b19f11119a664a02
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/libs/qmljs/qmljs.qbs

@@ -8,6 +8,7 @@ QtcLibrary {
     ])
     cpp.optimization: "fast"
 
+    Depends { name: "ExtensionSystem" }
     Depends { name: "Utils" }
     Depends { name: "LanguageUtils" }
     Depends { name: "CPlusPlus" }