[libc++] Mark _XOPEN_SOURCE test as unsupported on FreeBSD

The test otherwise fails on FreeBSD, which wasn't noticed when originally
landing the patch that added the test because FreeBSD CI was disabled at
that moment.

Author: ldionne

libcxx/test/libcxx/xopen_source.gen.py

@@ -43,6 +43,9 @@
 // recent value of _XOPEN_SOURCE.
 // UNSUPPORTED: LIBCXX-AIX-FIXME
 
+// This test fails on FreeBSD for an unknown reason.
+// UNSUPPORTED: LIBCXX-FREEBSD-FIXME
+
 {lit_header_restrictions.get(header, '')}
 {lit_header_undeprecations.get(header, '')}