platform: fix nm_platform_lnk_gre_to_string() for tap links

Why didn't we get a compiler warning about this bug?
At least clang (3.8.0-2ubuntu4, Ubuntu 16.04) warns:

    CC       src/platform/src_libNetworkManagerBase_la-nm-platform.lo
  ../src/platform/nm-platform.c:5389:14: error: data argument not used by format string [-Werror,-Wformat-extra-args]
                      lnk->remote ? nm_sprintf_buf (str_remote, " remote %s", nm_utils_inet4_ntop (lnk->remote, str_remote1)) : "",
                      ^

Fixes: 4c2862b9 ('platform: add gretap tunnels support')
(cherry picked from commit dfb899f4)
(cherry picked from commit ed88c71f)
(cherry picked from commit 400293d3)
4 jobs for nm-1-14 in 17 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #247276
allowed to fail
checkpatch

00:05:11

passed #247277
f28_build_default

00:06:59

passed #247278
f28_rpm_autotools_dbg

00:17:49

passed #247279
f28_rpm_meson_dbg

00:06:51