Include sys/rseq.h in tst-rseq-disable.c

Include sys/rseq.h in tst-rseq-disable.c

Starting with commit 2c6b4b272e6b4d07303af25709051c3e96288f2d
"nptl: Unconditionally use a 32-byte rseq area", the testcase
misc/tst-rseq-disable is UNSUPPORTED as RSEQ_SIG is not defined.

The mentioned commit removes inclusion of sys/rseq.h in nptl/descr.h.
Thus just include sys/rseq.h in the tst-rseq-disable.c as also done
in tst-rseq.c and tst-rseq-nptl.c.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
(cherry picked from commit 637aac2ae3980de31a6baab236a9255fe853cc76)