scripts/misc-check: add double-quotes to satisfy shellcheck

In scripts/misc-check line 8:
        git -C ${srctree:-.} ls-files -i -c --exclude-per-directory=.gitignore 2>/dev/null |
               ^-----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>