tests: Skip netlink policy test on non-Linux platforms.

FreeBSD tests in Cirrus CI are broken and, I guess, windows tests too:

  89. library.at:258: testing netlink policy ...
  ./library.at:259: ovstest test-netlink-policy ll_addr
  --- /dev/null	2021-08-20 19:02:41.907547000 +0000
  +++ /tmp/cirrus-ci-build/tests/testsuite.dir/at-groups/89/stderr
  @@ -0,0 +1 @@
  +ovstest: unknown command 'test-netlink-policy'; use --help for help
  ./library.at:259: exit code was 1, expected 0
  89. library.at:258: 89. netlink policy (library.at:258): FAILED

'tests/test-netlink-policy.c' is built only on Linux, test
must be skipped on all other platforms to unblock CI.

Fixes: bfee9f6 ("netlink: Add support for parsing link layer address.")
Signed-off-by: Ilya Maximets <[email protected]>
Acked-by: Frode Nordahl <[email protected]>

tests/library.at

@@ -256,5 +256,7 @@ AT_CHECK([ovstest test-stopwatch], [0], [......
 AT_CLEANUP
 
 AT_SETUP([netlink policy])
+AT_SKIP_IF([test "$IS_WIN32" = "yes"])
+AT_SKIP_IF([test "$IS_BSD" = "yes"])
 AT_CHECK([ovstest test-netlink-policy ll_addr], [0])
 AT_CLEANUP