CmbIpc: Fix wrong block size

The block size was one to big and so the command could only read after a
new command arrived.

Change-Id: I64ae47667d27d66e132031b10d4f130c53d3b0e9
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/codemodelbackendipc/writecommandblock.cpp

@@ -56,7 +56,7 @@ void WriteCommandBlock::write(const QVariant &command)
     out << command;
 
     out.device()->seek(0);
-    out << qint32(block.size());
+    out << qint32(block.size() - sizeof(qint32));
 
     ++commandCounter;