Skip 'maintenance check xml-descriptions' if XML is disabled

Skip 'maintenance check xml-descriptions' if XML is disabled

I see the following test failure when gdb is configured without XML
support,

maintenance check xml-descriptions binutils-gdb/gdb/testsuite/../features
warning: Can not parse XML target description; XML support was disabled at compile time^M
Tested 29 XML files, 29 failed
(gdb) FAIL: gdb.gdb/unittest.exp: maintenance check xml-descriptions ${srcdir}/../features

gdb/testsuite:

2017-12-15  Yao Qi  <yao.qi@linaro.org>

	* gdb.gdb/unittest.exp: Skip 'maintenance check xml-descriptions'
	if XML is disabled.