Compile fix for Heaps

Author: NotHyper-474

polymod/backends/HEAPSBackend.hx

@@ -3,7 +3,7 @@ package polymod.backends;
 import haxe.io.Bytes;
 import polymod.Polymod;
 import polymod.util.Util;
-import polymod.backends.PolymodAssetLibrary;
+import polymod.PolymodAssetLibrary;
 import polymod.PolymodAssets.PolymodAssetType;
 #if unifill
 import unifill.Unifill;