libstdc++: Disable PCH for test that depends on a macro being defined

libstdc++: Disable PCH for test that depends on a macro being defined

This test tries to ensure that <system_error> can be included after
defining _XOPEN_SOURCE=600, which doesn't test anything if that header
is already included via the <bits/stdc++.h> PCH before the macro
definition. Disable PCH so that it behaves as intended.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* testsuite/19_diagnostics/headers/system_error/93151.cc:
	Disable PCH.