netifd: ifup-shellscript - fix wrong usage of 'local'

netifd: ifup-shellscript - fix wrong usage of 'local'

this error was not visible until recent bump to
busybox 1.24.1 stable which introduced a warning message
when keyword 'local' is not used with a shell-function.

this does not change behavior and is a cosmetic cleanup.
fixes the following output:

root@box:~ ifup <interface>
/sbin/ifup: local: line 362: not in a function
/sbin/ifup: local: line 362: not in a function
/sbin/ifup: local: line 1: not in a function

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47828 3c298f89-4303-0410-b956-a3cf2f4a3e73