examples/ethtool: fix link with ixgbe shared lib

examples/ethtool: fix link with ixgbe shared lib

When RTE_DEVEL_BUILD is unset, -rpath is unset.
So the ethtool app cannot link with ixgbe shared library
which is required by ethtool lib:

warning: librte_pmd_ixgbe.so.1, needed by
examples/ethtool/lib/x86_64-native-linuxapp-gcc/lib/librte_ethtool.so,
not found (try using -rpath or -rpath-link)

It is fixed by adding the library in the application link.

The library link is also improved to specify that this explicit link
to ixgbe is needed only in the shared lib mode.

Fixes: 077d223e25c3 ("examples/ethtool: use ixgbe public function")

Signed-off-by: Markos Chandras <mchandras@suse.de>
Acked-by: Remy Horton <remy.horton@intel.com>
Acked-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>