Add wx/msw/panel.h to the list of wxMSW headers in bakefile.

Add wx/msw/panel.h to the list of wxMSW headers in bakefile.

Also move src/msw/panel.cpp to the list of normal wxMSW sources from the list
of low-level ones as it's not needed by wxUniv.

Closes #13103.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67357 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775