Fix build: for some reason, these includes became necessary

(Qt version here is kde/5.15 ~latest)

kcm/kcm.cpp

@@ -43,6 +43,7 @@
 #include <QQuickItem>
 #include <QQuickView>
 #include <QQuickWidget>
+#include <QStringBuilder>
 #include <QTimer>
 #include <QVBoxLayout>
 

libs/models/networkmodelitem.cpp

@@ -32,6 +32,8 @@
 #include <ModemManagerQt/ModemDevice>
 #endif
 
+#include <QStringBuilder>
+
 NetworkModelItem::NetworkModelItem(QObject *parent)
     : QObject(parent)
     , m_connectionState(NetworkManager::ActiveConnection::Deactivated)