import guard for report_xls

The dependency is indeed declared in __openerp__.py and will be
enforced. Odoo evaluates all python files, even for modules which are
not installed, and crashes if an unneeded dependency is needed.

mis_builder/report/__init__.py

@@ -22,5 +22,9 @@
 #
 ##############################################################################
 
-from . import mis_builder_xls
+try:
+    from . import mis_builder_xls
+except ImportError:
+    pass  # this module is not installed
+
 from . import report_mis_report_instance