qtoolbar: add missing header on OSX.

Without this you get:
```
widgets/qtoolbar.cpp:1047:5: error: incomplete type 'QPlatformNativeInterface' named in nested name specifier
    QPlatformNativeInterface::NativeResourceForIntegrationFunction function =
    ^~~~~~~~~~~~~~~~~~~~~~~~~~
qtbase/src/widgets/../../include/QtGui/../../src/gui/kernel/qguiapplication.h:57:7: note:
      forward declaration of 'QPlatformNativeInterface'
class QPlatformNativeInterface;
```

Committed upstream at https://github.com/qtproject/qtbase/commit/5d6b763ee521b830c2b19949016f9436627496d6