Add missing include to syncqt/main.cpp

I got build-time errors for memcpy() not being declared, that
helpfully told me to #include <cstring> to get it.

Change-Id: I6ae9e881e5accf496e9c3694ca43701972d64722
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/tools/syncqt/main.cpp

@@ -19,6 +19,7 @@
 #include <iostream>
 #include <fstream>
 #include <string>
+#include <cstring>
 #include <sstream>
 #include <filesystem>
 #include <unordered_map>