u32: add rtnl_u32_get_key()

Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>
build: fix build failure when using different build dir (make distcheck)

`make distcheck` builds outside the source directory using
`../configure --srcdir=..`. This was broken because libnl.sym was
expected in the $(top_srcdir) instead of $(top_builddir).

Signed-off-by: Thomas Haller <thaller@redhat.com>