Restore wxAnimation::GetFrame() constness

This got lost, almost surely accidentally, in 706c8e8ad6 (Merge branch
'disable-native-animation', 2020-04-07), so just restore it now.

See https://github.com/wxWidgets/wxWidgets/pull/1768

Closes #18725.

src/common/animatecmn.cpp

@@ -103,7 +103,7 @@ unsigned int wxAnimation::GetFrameCount() const
     return GetImpl()->GetFrameCount();
 }
 
-wxImage wxAnimation::GetFrame(unsigned int frame)
+wxImage wxAnimation::GetFrame(unsigned int frame) const
 {
     wxCHECK_MSG( IsOk(), wxNullImage, wxT("invalid animation") );
     return GetImpl()->GetFrame(frame);