iw: fix the Makefile for multiple libnl installations

iw: fix the Makefile for multiple libnl installations

All different versions of libnl can cohexist on a system
(libnl-1, libnl-2, libnl-3.0, libnl-3.1 and libnl-3.2).
When multiple versions are installed only link against
the highest versioned one, otherwise there might be
unpredictable results (including segmentation faults):

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7bd7f7d in genl_unregister () from /usr/lib64/libnl-genl.so.3
(gdb) where

This patch fixes the Makefile to link against only one
of the libraries, the highest version found.

Signed-off-by: Guido Trentalancia <guido@trentalancia.com>