stdio_filebuf is instantiated in the library but not exported

[Jonathan Wakely]

While working on iostream move semantics I noticed that we define an
explicit instantiation of __gnu_ext::stdio_filebuf<char> and
__gnu_ext::stdio_filebuf<wchar_t> in src/c++98/ext-inst.cc (which I
moved to src/c++11/ext11-inst.cc yesterday).

But we don't declare those explicit instantiations in any header, and
don't export the symbols, so the instantiations are only for the
library's own use, is that indented?