Fix build of command-line qbs with qbs

The plugin path we pass as a define to the qbs sources contained the "/
qbs/plugins" suffix which qbs already adds itself. Therefore, the
command-line qbs was not functional.

Change-Id: I2bfb967f998e69bbe4da760a1f517e1f40e4f055
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

qbs/modules/qbsbuildconfig/qbsbuildconfig.qbs

@@ -25,7 +25,7 @@ Module {
     property string relativeLibexecPath: FileInfo.relativePath('/' + appInstallDir,
                                                                '/' + libexecInstallDir)
     property string relativePluginsPath: FileInfo.relativePath('/' + appInstallDir,
-                                                               '/' + pluginsInstallDir)
+                                                               '/' + qtc.ide_plugin_path)
     property string relativeSearchPath: FileInfo.relativePath('/' + appInstallDir,
                                                               '/' + resourcesInstallDir)
 }