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;
```

Change-Id: I1301a8aa8b25eb12821e91125743be779e52db0f
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Mike McQuaid <mike@mikemcquaid.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>