net/mlx: fix overlinking with meson and glue dlopen

net/mlx: fix overlinking with meson and glue dlopen

If ibverbs_link is dlopen, the PMD and application should not
be linked with ibverbs, but the glue library is.
Unfortunately the ibverbs dependency was exported in the
variable ext_deps, so there were overlinking.

It is fixed by not exporting the dependency in ext_deps,
and recreating a limited dependency object for cflags only.

Fixes: 1dd7c7e38c19 ("net/mlx4: support meson build")
Fixes: 96d7c62a70c7 ("net/mlx5: support meson build")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>