libnm: when stringifying policy routing rule place "not" specifier after "priority"

Otherwise, it just looks odd:

  "not priority 31265 from 0.0.0.0/0 fwmark 0xcb87 table 52103"

better would be

  "priority 31265 not from 0.0.0.0/0 fwmark 0xcb87 table 52103"

The "not" specifier should come after the priority. It makes more sense
to read it that way. As far as parsing the string is concerned, the
order does not matter. So this change in behavior is no problem.
15 jobs for th/policy-route-to-string-not-attribute in 34 minutes and 32 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #469456
allowed to fail
checkpatch

00:00:58

manual #469461
allowed to fail manual
t_centos:7.5.1804
passed #469462
t_centos:7.6.1810

00:34:31

passed #469468
t_debian:10

00:10:52

passed #469467
t_debian:9

00:25:12

passed #469470
t_debian:sid

00:14:05

manual #469469
allowed to fail manual
t_debian:testing
passed #469457
t_fedora:28

00:23:32

passed #469458
t_fedora:29

00:31:19

passed #469459
t_fedora:30

00:25:13

manual #469460
allowed to fail manual
t_fedora:rawhide
passed #469463
t_ubuntu:16.04

00:14:07

passed #469464
t_ubuntu:18.04

00:26:40

manual #469466
allowed to fail manual
t_ubuntu:devel
manual #469465
allowed to fail manual
t_ubuntu:rolling