Force --headless argument when LIBO_HEADLESS is defined

When LIBO_HEADLESS is set we are not supposed to run libo without
--headless; doing so will throw an UNO exception. So better use
a sensible default.

desktop/source/app/cmdlineargs.cxx

@@ -588,7 +588,11 @@ void CommandLineArgs::InitParamValues()
     m_minimized = false;
     m_invisible = false;
     m_norestore = false;
+#ifdef LIBO_HEADLESS
+    m_headless = true;
+#else
     m_headless = false;
+#endif
     m_quickstart = false;
     m_noquickstart = false;
     m_terminateafterinit = false;