Always define WXUSINGDLL when compiling Scintilla in shared wx build.

Always define WXUSINGDLL when compiling Scintilla in shared wx build.

We need WXUSINGDLL even in monolithic build because Scintilla references wx
debugging functions (wxOnAssert(), wxTheAssertHandler &c) which still must be
seen as being exported from the (monolithic) DLL in this case.

See #12626.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66222 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775