We do want to initialise m_MainThread on iOS, too

This doesn't make the LibreOfficeLight app work much better, though.
But at least it doesn't hang its event loop completely, some 10 s
timer keeps firing. (But nothing sane shows up in the app UI, even if
some welcome.odt document apparently does get loaded. Clearly
something is waiting for something else that never happens...)

Change-Id: Ieba22a5e1418d48a7dd6cacda526aca69cced4c3

vcl/headless/svpinst.cxx

@@ -70,8 +70,8 @@ SvpSalInstance::SvpSalInstance( std::unique_ptr<SalYieldMutex> pMutex )
     m_aTimeout.tv_usec      = 0;
     m_nTimeoutMS            = 0;
 
-#ifndef IOS
     m_MainThread = osl::Thread::getCurrentIdentifier();
+#ifndef IOS
     CreateWakeupPipe(true);
 #endif
     if( s_pDefaultInstance == nullptr )