Only run the spec on Linux

* Other platforms seem to behave differently.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64499 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/socket/socket/accept_spec.rb

@@ -12,7 +12,7 @@ describe 'Socket#accept' do
       @server.close unless @server.closed?
     end
 
-    platform_is_not :aix, :darwin, :freebsd, :solaris, :windows do # hangs
+    platform_is :linux do # hangs on other platforms
       describe 'using an unbound socket'  do
         it 'raises Errno::EINVAL' do
           lambda { @server.accept }.should raise_error(Errno::EINVAL)