lock the StyleBucket to avoid crashing

stopgap until we have a solution that doesn't mutate the style objects while parsing a tile

src/mbgl/map/vector_tile_data.cpp

@@ -4,6 +4,7 @@
 #include <mbgl/map/map.hpp>
 #include <mbgl/style/style_layer.hpp>
 #include <mbgl/style/style_bucket.hpp>
+#include <mbgl/style/style_source.hpp>
 #include <mbgl/geometry/glyph_atlas.hpp>
 #include <mbgl/platform/log.hpp>