All Truffle::POSIX types should be Symbols

Author: eregon

src/main/ruby/truffleruby/core/posix.rb

@@ -85,7 +85,7 @@ def self.to_nfi_type(type)
   end
 
   def self.varargs(type)
-    "...#{to_nfi_type(type)}"
+    :"...#{to_nfi_type(type)}"
   end
 
   # the actual function is looked up and attached on its first call