fix inclusion of functions.sh in dsl_fs init script

without this, the following error is seen during build:
./etc/init.d/dsl_fs: line 4: /lib/functions.sh: No such file or directory

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>

Backport of r42575

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42580 3c298f89-4303-0410-b956-a3cf2f4a3e73