include the gtk3 generated ui file when building against gtk3. This way both flavors build

src/process-window.c

@@ -22,7 +22,11 @@
 
 #include "settings.h"
 #include "process-window.h"
+#ifdef HAVE_GTK3
+#include "process-window-gtk3_ui.h"
+#else
 #include "process-window_ui.h"
+#endif
 #include "process-monitor.h"
 #include "process-tree-view.h"
 #include "process-statusbar.h"

src/settings-dialog.c

@@ -17,7 +17,11 @@
 
 #include "settings.h"
 #include "settings-dialog.h"
+#ifdef HAVE_GTK3
+#include "settings-dialog-gtk3_ui.h"
+#else
 #include "settings-dialog_ui.h"
+#endif