Fix an import regresion

Restored a required import; without it, oq-engine breaks.

It causes this test failure: 
<pre>
test_classical_hazard_export (tests.export.hazard_test.ClassicalExportTestCase) ... ERROR

======================================================================
ERROR: test_classical_hazard_export (tests.export.hazard_test.ClassicalExportTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ubuntu/oq-engine/tests/export/hazard_test.py", line 313, in test_classical_hazard_export
    hc_files.append(check_export(curve.id, target_dir))
  File "/home/ubuntu/oq-engine/tests/export/hazard_test.py", line 39, in check_export
    out_file = hazard.export(output_id, target, 'xml')
  File "/home/ubuntu/oq-engine/openquake/engine/export/hazard.py", line 64, in export
    return export_fn(output, target)
  File "/home/ubuntu/oq-engine/openquake/engine/export/core.py", line 55, in wrapped
    return fn(output, target)
  File "/home/ubuntu/oq-engine/openquake/engine/export/hazard.py", line 206, in export_hazard_curve_xml
    nrml.HazardCurveXMLWriter(dest, **metadata).serialize(hcd)
AttributeError: 'module' object has no attribute 'HazardCurveXMLWriter'
</pre>