[FIX][report_xls] Protect xlwt import.

Given `xlwt` could happen to not be installed because it is an optional dependency.

See https://github.com/odoo/odoo/issues/12897#issuecomment-234928120.

report_xls/report_xls.py

@@ -20,8 +20,6 @@
 #
 ##############################################################################
 
-import xlwt
-from xlwt.Style import default_style
 import cStringIO
 from datetime import datetime
 from openerp.osv.fields import datetime as datetime_field
@@ -33,6 +31,12 @@ from openerp import pooler
 import logging
 _logger = logging.getLogger(__name__)
 
+try:
+    import xlwt
+    from xlwt.Style import default_style
+except ImportError:
+    _logger.debug("Cannot import xlwt. This module will not be functional.")
+
 
 class AttrDict(dict):
     def __init__(self, *args, **kwargs):