WebAssembly: Fix run configuration for Qt 5.15 app with emsdk 1.39.x

With Emscripten SDK 1.39.x, emrun quits and shuts down the web server
before the complete WebAssembly application can be fetched by the
browser.

Adding "--serve_after_close" to the emrun call restores the original
behavior. That parameter is accepted by previous Emscripten SDK versions
and seems to not change how it used behave in earlier versions.

Task-number: QTCREATORBUG-24072
Change-Id: Ie43604d42eb9c5a7cccd09fdf1c33391dc31098d
Reviewed-by: hjk <hjk@qt.io>

src/plugins/webassembly/webassemblyrunconfiguration.cpp

@@ -51,6 +51,7 @@ static CommandLine emrunCommand(Target *target, const QString &browser, const QS
                 "--browser", browser,
                 "--port", port,
                 "--no_emrun_detect",
+                "--serve_after_close",
                 html.toString()
             });
     }