[ios] Include offline headers in static umbrella header

Include offline headers in the umbrella header used by the static framework. These includes are already present in the umbrella header used by the dynamic framework. jazzy relies on the static umbrella header to include all the headers, so the offline headers were also missing from documentation.

.jazzy.yaml

@@ -37,7 +37,7 @@ custom_categories:
       - MGLOverlay
       - MGLShape
       - MGLUserLocation
-  - name: Offline Viewing
+  - name: Offline Storage
     children:
       - MGLOfflineRegion
       - MGLOfflineStorage

platform/ios/include/Mapbox.h

@@ -8,11 +8,15 @@
 #import "MGLMapView+IBAdditions.h"
 #import "MGLMapView+MGLCustomStyleLayerAdditions.h"
 #import "MGLMultiPoint.h"
+#import "MGLOfflinePack.h"
+#import "MGLOfflineRegion.h"
+#import "MGLOfflineStorage.h"
 #import "MGLOverlay.h"
 #import "MGLPointAnnotation.h"
 #import "MGLPolygon.h"
 #import "MGLPolyline.h"
 #import "MGLShape.h"
 #import "MGLStyle.h"
+#import "MGLTilePyramidOfflineRegion.h"
 #import "MGLTypes.h"
 #import "MGLUserLocation.h"