kmerge.sh: Fix line wrapping typo from 9ceebbf

kmerge.sh: Fix line wrapping typo from 9ceebbf

In 9ceebbf (kmerge.sh: Make /var/tmp/${clst_kname}.config optional,
2013-02-09), I added a KERNCACHE check to a `[` test, wrapping the two
clauses in the test to avoid a very long line.  Unfortunately,
newlines do not appear to be legal in this position.  For example:

  $ if [ -n "a" -a
  > -n "b" ]; then echo "c"; fi
  bash: [: missing `]'
  bash: -n: command not found

This commit fixes the error by explicitly wrapping the line with a
backslash.

Reviewed-by: Matt Turner <mattst88@gmail.com>