[ruby/json] Fix --with-static-linked-ext builds

ruby/json@d7d60cccb0

Author: byroot

ext/json/generator/extconf.rb

@@ -35,7 +35,5 @@
     have_header('cpuid.h')
   end
 
-  create_header
-
   create_makefile 'json/ext/generator'
 end