Freeze NativeCommon for Ractor

Only used while testing the FFI backend on CRuby.

Author: headius

lib/prism/ffi/native_ffi.rb

@@ -320,6 +320,6 @@ def string_query_local(string)
     end
   end
 
-  FFICommon = NativeCommon.new
+  FFICommon = NativeCommon.new.freeze
   private_constant(:FFICommon)
 end