net/safe-search: Fix double installation bug

net/safe-search: Fix double installation bug

If safe search is built directly into an image, the /etc/config/dhcp
file will have multiple entries added to it after using sysupgrade
for the nth time (2 or more sysupgrade cycles).

In /etc/config/dhcp, this bug creates duplicate entries like this:

config dnsmasq
	list addnhosts '/etc/safe-search/enabled'
	list addnhosts '/etc/safe-search/enabled'

This patch ensures that safe search only registers itself one time.

Signed-off-by: Gregory L. Dietsche <gregory.dietsche@cuw.edu>