package/iputils: use relative symlink for ping6

package/iputils: use relative symlink for ping6

Fixes (part of):
http://autobuild.buildroot.net/results/5659e1c91831921bd9ad6af670258783771b4dc8/

Commit 6b37dda2a972cdf (package/iputils: create ping6 symlink), added a
ping6 symlink, but used the absolute (build) path to ping as the target,
which is naturally no good at runtime:

ls -l target/bin/ping6
lrwxrwxrwx 1 peko peko 58 Jan 10 08:25 target/bin/ping6 -> /home/peko/source/buildroot/output-iputils/target/bin/ping

Instead use a relative symlink.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>