Do not warn of missing PT_GNU_RELRO segment when custom DSO filter used

Do not warn of missing PT_GNU_RELRO segment when custom DSO filter used

It is most likely that it is not found just because the segment had
been excluded.

Alternatively, we could have registered all segments, and checked the
callback afterwards, but then we could break programs that rely on
GC_has_static_roots_func to avoid overflowing the maximum number of
roots.  In addition, it would make the logic slightly more
complicated, probably without a very good reason since the chance of
this warning to show without the segment being excluded is likely to
be none.

* dyn_load.c [HAVE_DL_ITERATE_PHDR] (GC_register_dynlib_callback):
Do not call WARN() if GC_has_static_roots callback is set.