Check for wxUSE_ACTIVITYINDICATOR in the widgets sample.

This fixes the sample compilation with default configuration when using MinGW
as it uses wxUSE_GRAPHICS_CONTEXT=0 by default and so wxUSE_ACTIVITYINDICATOR
is turned off by default for it too.

samples/widgets/activityindicator.cpp

@@ -23,6 +23,8 @@
     #pragma hdrstop
 #endif
 
+#if wxUSE_ACTIVITYINDICATOR
+
 // for all others, include the necessary headers
 #ifndef WX_PRECOMP
     #include "wx/button.h"
@@ -149,3 +151,5 @@ void ActivityIndicatorWidgetsPage::RecreateWidget()
     m_sizerIndicator->AddStretchSpacer();
     m_sizerIndicator->Layout();
 }
+
+#endif // wxUSE_ACTIVITYINDICATOR