Commit fe5527f1 (desktop, sysui: let
--convert-to imply --headless, 2014-12-22) changed
CommandLineArgs::ParseCommandLine_Impl(), so that --convert-to implies
--headless. However, that worked only when ./soffice.bin was launched;
when the ./soffice wrapper started oosplash, then a logo was still
shown, as oosplash has its own list of switches that disable the splash
screen.

Fix the inconsistency by adding "convert-to" to oosplash's list as well.

Change-Id: I47e30db3b658f179f10e8a447b5d7cea083fcc60