ddns-scripts: no longer build public_suffix_list.dat.gz during build
- no longer build public_suffix_list.dat.gz during build #3678
- replace "\s" with "[[:space:]]" inside Makefile because "\s" ignored by some sed versions
- tools/public_suffix_list.sh still available to rebuild public_suffix_list.dat.gz outside OpenWRT/LEDE build system
Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>