After r232548, clang complains about the apparent '=-' operator (a

After r232548, clang complains about the apparent '=-' operator (a
left-over from ancient C times, and a frequent typo) in growfs.c:

sbin/growfs/growfs.c:1550:8: error: use of unary operator that may be intended as compound assignment (-=) [-Werror]
        blkno =- 1;
              ^~

Use 'blkno = -1' instead, to silence the error.