Always link with expat in monolithic build.

Always link with expat in monolithic build.

Fix linking errors when using wxXML in sttaic monolithic build: in this case
we must link with expat but it was listed too early, before wxWidgets
libraries using it, in the bakefile-generated makefiles.

Now add it to the end of the linker command line too by including -lexpat in
EXTRALIBS_FOR_BASE which are appended to the linker arguments.

Notice that now -lexpat is linked twice because it is still added by
<wx-lib>xml</wx-lib> in bakefiles, even in monolithic build, but I don't know
how to avoid it and it is harmless anyhow.

Closes #12573.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70116 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775