spec/ruby/library/rbconfig/rbconfig_spec.rb: restore "not windows" guard

https://github.com/ruby/ruby/runs/541455267
```
1)
RbConfig::CONFIG libdir/LIBRUBY_SO is the path to libruby and it exists if and only if ENABLE_SHARED FAILED
Expected File.exist? "d:/Ruby26-x64/lib/x64-msvcrt-ruby260.dll"
to be truthy but was false
D:/a/ruby/ruby/src/spec/ruby/library/rbconfig/rbconfig_spec.rb:46:in `block (3 levels) in <top (required)>'
D:/a/ruby/ruby/src/spec/ruby/library/rbconfig/rbconfig_spec.rb:4:in `<top (required)>'
```

spec/ruby/library/rbconfig/rbconfig_spec.rb

@@ -37,15 +37,17 @@ describe 'RbConfig::CONFIG' do
     RUBY
   end
 
-  guard -> {RbConfig::TOPDIR} do
-    it "libdir/LIBRUBY_SO is the path to libruby and it exists if and only if ENABLE_SHARED" do
-      libdir = RbConfig::CONFIG[RbConfig::CONFIG['libdirname'] || 'libdir']
-      libruby_so = "#{libdir}/#{RbConfig::CONFIG['LIBRUBY_SO']}"
-      case RbConfig::CONFIG['ENABLE_SHARED']
-      when 'yes'
-        File.should.exist?(libruby_so)
-      when 'no'
-        File.should_not.exist?(libruby_so)
+  platform_is_not :windows do
+    guard -> {RbConfig::TOPDIR} do
+      it "libdir/LIBRUBY_SO is the path to libruby and it exists if and only if ENABLE_SHARED" do
+        libdir = RbConfig::CONFIG[RbConfig::CONFIG['libdirname'] || 'libdir']
+        libruby_so = "#{libdir}/#{RbConfig::CONFIG['LIBRUBY_SO']}"
+        case RbConfig::CONFIG['ENABLE_SHARED']
+        when 'yes'
+          File.should.exist?(libruby_so)
+        when 'no'
+          File.should_not.exist?(libruby_so)
+        end
       end
     end
   end