bus/pci: remove useless link dependency on ethdev

bus/pci: remove useless link dependency on ethdev

The makefile in drivers/bus/pci specified rte_ethdev as a dependency for
the library. However there are no actual symbols from librte_ethdev used
in librte_bus_pci.

Including librte_ethdev as a dependency only becomes a problem in some
niche cases like when attempting to build the rte_bus_pci library as a
shared object without building the rte_ethdev library.

I specifically ran into this when trying to build the DPDK included as
an SPDK submodule on a FreeBSD machine. I figure that since there are no
real dependencies between the two, we should enable building
librte_bus_pci without librte_ethdev.

Fixes: c752998b5e2e ("pci: introduce library and driver")
Cc: stable@dpdk.org
Signed-off-by: Seth Howell <seth.howell@intel.com>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>