Use correct Unicode define for wxScintilla compilation.

Compilation of wxScintilla with Borland failed because -D_UNICODE was not
passed on command line resulting in mismatches between wxChar and Windows
TCHAR. It's a mystery why this didn't happen with the other compilers but
defining _UNICODE for them too can't hurt.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65477 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775