XFAIL a test in g++.dg/warn/Wstringop-overflow-4.C

XFAIL a test in g++.dg/warn/Wstringop-overflow-4.C

As per the explanation in the test, and in the DOM conversion to
ranger patch, this is a known regression.  I had mentioned I would
XFAIL this test, but forgot to do so.  There is an analysis in the
test itself as to what is going on.

Tested on x86-64 Linux.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wstringop-overflow-4.C: XFAIL a test.