mk: fix dependencies to optional configs

mk: fix dependencies to optional configs

In rte.lib.mk, the list of libraries passed to the link
command (LDLIBS) is generated from the DEPDIRS-xxx variables.
If a library is not compiled because it is disabled in
configuration, it should not appear in DEPDIRS-xxx.

- librte_port depends on librte_kni only if it is enabled.
- librte_table depends on librte_acl only if it is enabled.

Fixes: feb9f680cd2c ("mk: optimize directory dependencies")
Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>