msi: Set concurrency model to multi-threaded for custom action threads.

A custom action in the office 2007 sp1 installer fails right after calling
CoInitializeEx(NULL, COINIT_MULTITHREADED). This is because our code calls
CoInitialize(NULL) first, which sets a different concurrency model, and a
subsequent call with a different model will return RPC_E_CHANGED_MODE.

Ergo, we should set the model to multi-threaded too.

dlls/msi/custom.c

@@ -695,7 +695,7 @@ static DWORD ACTION_CallDllFunction( const GUID *guid )
             TRACE("calling %s\n", debugstr_w( function ) );
             handle_msi_break( function );
 
-            CoInitialize(NULL);
+            CoInitializeEx(NULL, COINIT_MULTITHREADED);
 
             __TRY
             {