Skip parse_memory_map_tests if XML is disabled

Skip parse_memory_map_tests if XML is disabled

I find a fail in gdb unit test when gdb is configured without XML
support.

warning: Can not parse XML memory map; XML support was disabled at compile time^M
Self test failed: self-test failed at ../../binutils-gdb/gdb/unittests/memory-map-selftests.c:65
...
Ran 31 unit tests, 1 failed^M
(gdb) FAIL: gdb.gdb/unittest.exp: maintenance selftest

gdb:

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

	* unittests/memory-map-selftests.c: Wrap test with HAVE_LIBEXPAT.