UI: Fix missing includes

These files appeared to be dependent on their include order and were
missing definitions when files were compiled in a different order.

UI/properties-view.moc.hpp

@@ -6,6 +6,7 @@
 #include <QStackedWidget>
 #include <QWidget>
 
+#include <obs.h>
 #include <media-io/frame-rate.h>
 
 #include <vector>

UI/source-tree.hpp

@@ -8,6 +8,8 @@
 #include <QStaticText>
 #include <QSvgRenderer>
 #include <QAbstractListModel>
+#include <obs.hpp>
+#include <obs-frontend-api.h>
 
 class QLabel;
 class QCheckBox;

UI/window-basic-stats.hpp

@@ -2,6 +2,7 @@
 
 #include <obs.hpp>
 #include <util/platform.h>
+#include <obs-frontend-api.h>
 #include <QPointer>
 #include <QWidget>
 #include <QTimer>