Fix compile without built-in qsqlite

src/core/main.cpp

@@ -82,8 +82,10 @@
 #endif
 
 // Load sqlite plugin on windows and mac.
+#ifndef USE_SYSTEM_QSQLITE
 #include <QtPlugin>
 Q_IMPORT_PLUGIN(QSQLiteDriverPlugin)
+#endif
 
 int main(int argc, char* argv[]) {