libstdc++: Fix <iosfwd> synopsis test

The <syncstream> header is only supported for the cxx11 ABI. The
declarations of basic_syncbuf, basic_osyncstream, syncbuf and
osyncstream were already correctly guarded by a check for
_GLIBCXX_USE_CXX11_ABI, but the wsyncbuf and wosyncstream declarations
were not.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/headers/iosfwd/synopsis.cc: Make wsyncbuf and
	wosyncstream depend on _GLIBCXX_USE_CXX11_ABI.