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

build/msw/makefile.gcc

@@ -1532,11 +1532,9 @@ endif
 ifeq ($(USE_GUI),1)
 __wxtiff___depname = $(LIBDIRNAME)\libwxtiff$(WXDEBUGFLAG).a
 endif
-ifeq ($(MONOLITHIC),0)
 ifeq ($(SHARED),1)
 __wxscintilla_usingdll_p = -DWXUSINGDLL
 endif
-endif
 ifeq ($(MONOLITHIC),0)
 EXTRALIBS_FOR_BASE = 
 endif