select() on all platforms for Socket#recvmsg_nonblock spec

* The specs above already do that, and Solaris needs it too:
  https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable10x/ruby-trunk/log/20180819T111806Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64470 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/socket/basicsocket/recvmsg_nonblock_spec.rb

@@ -144,7 +144,7 @@ describe 'BasicSocket#recvmsg_nonblock' do
             @client.write('hello')
 
             @socket, _ = @server.accept
-            platform_is(:darwin, :freebsd) { IO.select([@socket]) }
+            IO.select([@socket])
           end
 
           after do