Initialize the atomic_flag in the class definition

ATOMIC_FLAG_INIT is often defined to { 0 } or { false } and some
compilers might complain about initializing as flag(ATOMIC_FLAG_INIT).

src/mbgl/map/tile_data.cpp

@@ -13,7 +13,6 @@ TileData::TileData(const TileID& id_, const SourceInfo& source_)
     : id(id_),
       name(id),
       state(State::initial),
-      parsing(ATOMIC_FLAG_INIT),
       source(source_),
       env(Environment::Get()),
       debugBucket(debugFontBuffer) {