Treat RSTRING_END return value as a char* instead of string

`RSTRING_END` expects pointer to return
https://github.com/ruby/ruby/blob/v3_4_2/include/ruby/internal/core/rstring.h#L434-L442

Author: sue445

config/default.yml.erb

@@ -81,6 +81,8 @@ function:
     <% end %>
 
   pointer_hint:
+    RSTRING_END:
+      self: raw
     RSTRING_PTR:
       self: raw
     rb_data_object_make:

spec/ruby_header_parser/parser_spec.rb

@@ -310,7 +310,7 @@
 
       its(:name)       { should eq "RSTRING_END" }
       its(:definition) { should eq "RSTRING_END(VALUE str)" }
-      its(:typeref)    { should eq typeref(type: "char", pointer: :ref) }
+      its(:typeref)    { should eq typeref(type: "char", pointer: :raw) }
       its(:args)       { should eq args }
     end