mlx4: fix shared library dependency

mlx4: fix shared library dependency

librte_pmd_mlx4.so needs to be linked with libibverbs otherwise, the PMD is
not able to open Mellanox devices and the following message is printed by
testpmd at startup
"librte_pmd_mlx4: cannot access device, is mlx4_ib loaded?".

Applications dependency on libibverbs are moved to be only valid in static
mode, in shared mode, applications do not depend on it anymore,
librte_pmd_mlx4.so keeps this dependency and thus is linked with libibverbs.

MLX4 cannot be supported in combined shared library because there is no clean
way of adding -libverbs to the combined library.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>