[FIX][report_xls] Protect xlwt import.
[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.

open-synergy/reporting-engine,syci/reporting-engine,eezee-it/reporting-engine,diagramsoftware/reporting-engine,brain-tec/reporting-engine,eezee-it/reporting-engine,eezee-it/reporting-engine,acsone/reporting-engine,diagramsoftware/reporting-engine,open-synergy/reporting-engine,ddico/reporting-engine,acsone/reporting-engine,diagramsoftware/reporting-engine,ddico/reporting-engine,ddico/reporting-engine,open-synergy/reporting-engine,acsone/reporting-engine,eezee-it/reporting-engine,brain-tec/reporting-engine,syci/reporting-engine,ddico/reporting-engine,brain-tec/reporting-engine,acsone/reporting-engine,open-synergy/reporting-engine,brain-tec/reporting-engine,diagramsoftware/reporting-engine