[webview] Missing Config include

This was causing CI builds to fail, since WEBVIEW_WITH_WEBKIT
is defined only in the Config file, not on the command-line.
This crept in accidentally while trying to get rid of that
config file entirely.

src/modules/webview/WebViewStep.cpp

@@ -17,6 +17,7 @@
  *   along with Calamares. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "WebViewConfig.h"
 #include "WebViewStep.h"
 
 #include <QVariant>