prelude.c.tmpl: get rid of errors with old gcc
* template/prelude.c.tmpl: moved diagnostic pragmas outside
prelude_eval() for very old gcc. [ruby-core:84449] [Bug #14234]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@61496 b2dd03c8-39d4-4d8f-98ff-823fe69b080e