Enable rthreads support on OpenBSD/powerpc.

gdb/ChangeLog:

        * ppcobsd-nat.c: Include "obsd-nat.h".
        (_initialize_ppcobsd_nat): Call obsd_add_target instead of
        add_target.
        * config/powerpc/obsd.mh (NATDEPFILES): Add obsd-nat.o.
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