internal.h: WARN_UNUSED_RESULT

* internal.h (WARN_UNUSED_RESULT): warn unused result by gcc 3.4
  or later.

* symbol.c: declare some functions with WARN_UNUSED_RESULT.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47323 b2dd03c8-39d4-4d8f-98ff-823fe69b080e