fib: fix headers for C++ support

fib: fix headers for C++ support

This patch allows C++ programs to use librte_fib library routines.
Previously C++ programs were not able to link this library and programs
would fail to get linked. With this patch compilation and linking
will be successful.

Fixes: 39e927248416 ("fib: add FIB library")
Fixes: 40d41a8a7b34 ("fib: support IPv6")
Cc: stable@dpdk.org
Signed-off-by: Asim Jamshed <asim.jamshed@gmail.com>
Signed-off-by: Saikrishna Edupuganti <saikrishna.edupuganti@intel.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>