xrCore/FS (IReader): Fixed close() compilation.

There was a deletion call xr_delete(this) and xr_delete() tries to
set passed argument (this) to NULL, or accept a pointer to const,
which is not the case either.

Used the same thing, as in other close() methods:
introduced variable self, which is passed to xr_delete().

src/xrCore/FS.cpp

@@ -362,7 +362,8 @@ IReader* IReader::open_chunk(u32 ID)
 };
 void IReader::close()
 {
-    xr_delete((IReader*)this);
+    IReader *self = this;
+    xr_delete(self);
 }
 
 #include "FS_impl.h"