Makefile: if symlink exists overwrite

Error if make install is done a second time e.g. after updating via git pull.
Pass -f to overwrite link to avoid this.

ln -s /usr/local/mal-dnssearch/mal-dnssearch.sh /usr/bin/mal-dnssearch
ln: failed to create symbolic link `/usr/bin/mal-dnssearch': File exists
make: *** [install] Error 1

ln -s /usr/local/mal-dnssearch/mal-dns2bro.sh /usr/bin/mal-dns2bro
ln: failed to create symbolic link `/usr/bin/mal-dns2bro': File exists
make: *** [install] Error 1

[Reported by DigiAngel]