Utils: Replace mutating FileName::appendString

... by a non-mutating .stringAppended, doing the same.

Change-Id: I7adb6cae3415942cc9a80088bd75cda9d577d4a5
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/android/androidmanager.cpp

@@ -286,9 +286,9 @@ QJsonObject AndroidManager::deploymentSettings(const Target *target)
     settings["useLLVM"] = true;
     settings["ndk-host"] = AndroidConfigurations::currentConfig().toolchainHost();
     settings["stdcpp-path"] = AndroidConfigurations::currentConfig().ndkLocation()
-            .appendPath("/sources/cxx-stl/llvm-libc++/libs/")
-            .appendString(targetArch(target))
-            .appendPath("libc++_shared.so").toString();
+            .appendPath("/sources/cxx-stl/llvm-libc++/libs/"
+                        + targetArch(target)
+                        + "/libc++_shared.so").toString();
     return settings;
 }