qbs build: Suppress vcs header file creation

The repository state is only used on the project file level at the
moment.

Change-Id: I8f6ff587785d4fbc5586836a31c256a2e114cdb6
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

qbs/modules/pluginjson/pluginjson.qbs

@@ -13,6 +13,7 @@ Module {
     Depends { name: "vcs"; condition: hasVcs }
     Properties {
         condition: hasVcs
+        vcs.headerFileName: undefined
         vcs.repoDir: {
             // TODO: Could something like this be incorporated into the vcs module?
             //       Currently, the default repo dir is project.sourceDirectory, which