Fix build regressions introduced by illumos #3604

The following errors were printed by GCC 4.5.3 on Gentoo Linux:

../../cmd/zdb/zdb.c: In function 'dump_bpobj':
../../cmd/zdb/zdb.c:1257:3: error: 'for' loop initial declarations are
only allowed in C99 mode
../../cmd/zdb/zdb.c:1257:3: note: use option -std=c99 or -std=gnu99 to
compile your code
../../cmd/zdb/zdb.c: In function 'dump_deadlist':
../../cmd/zdb/zdb.c:1323:8: warning: too many arguments for format
../../cmd/zdb/zdb.c:1323:8: warning: too many arguments for format

Signed-off-by: Richard Yao <ryao@gentoo.org>