Add prism/token_type.c to gitignore temporarily

It just keeps popping up in various branches when you rebase them. I
have just accidentally committed it in one of my PRs. Let me add it to
gitignore until enough time has passed since its deletion.

Author: k0kubun

.gitignore

@@ -276,6 +276,9 @@ lcov*.info
 /prism/srcs.mk
 /dump_ast
 
+# prism (old file)
+/prism/token_type.c
+
 # tool/update-NEWS-gemlist.rb
 /bundled_gems.json
 /default_gems.json