Xcursor: Don't ignore unknown file types

XFS doesn't return the file type with opendir(), and instead only
returns it when you stat() the file itself.  c66f2602 introduced a check
to ensure that only files and symlinks were loaded, while not checking
for DT_UNKNOWN.  Properly handle DT_UNKNOWN by just proceeding anyway.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>