one more ffs build break

[Paul Goyette <paul%whooppee.com@localhost>]

cc1: warnings being treated as errors
/build/netbsd-local/src/sbin/resize_ffs/resize_ffs.c: In function 'markblk':
/build/netbsd-local/src/sbin/resize_ffs/resize_ffs.c:1057:7: error: comparison 
between signed and unsigned integer expressions


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