Only define WXUSINGDLL in multilib builds for wxscintilla.lib.

Only define WXUSINGDLL in multilib builds for wxscintilla.lib.

In monolithic builds wxscintilla is linked directly into the one and only wx
DLL and doesn't need to import anything from it, WXUSINGDLL is only needed in
multilib case when wxscintilla is part of wxCore DLL and does need to import
symbols (e.g. wxQsort() used by wxVector) from wxBase one.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65109 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775