Fix "'RB_ROOT' undeclared" error on OpenBSD

Fix "'RB_ROOT' undeclared" error on OpenBSD

OpenBSD has below compiler error due to RB_ROOT name conflict between
lib/rbtree.h and OpenBSD header. os/os-{net,open}bsd.h actually have
had a workaround for this by undefing their RB_XXX macros (which fio
doesn't use) for possible conflicts, but then "lib/rbtree.h" needs
to be included after os/os.h which includes os/os-openbsd.h.

t/dedupe.c: In function 'main':
t/dedupe.c:583: error: 'RB_ROOT' undeclared (first use in this function)
t/dedupe.c:583: error: (Each undeclared identifier is reported only once
t/dedupe.c:583: error: for each function it appears in.)

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>