Fix possible compiler warning and linker error

Type was forward declared as class, but is a struct. GCC is quiet about this
Clang gives a warning about potential linker errors on MSVC.

source/blender/asset_system/AS_asset_library.hh

@@ -19,7 +19,6 @@
 #include "BKE_callbacks.h"
 
 struct AssetLibrary;
-class bUserAssetLibrary;
 struct IDRemapper;
 struct Main;