ruby.h: suppress warnings

* include/ruby/ruby.h (RUBY_SAFE_LEVEL_CHECK): suppress warnings
  "left-hand operand of comma expression has no effect", on gcc 4.4.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41292 b2dd03c8-39d4-4d8f-98ff-823fe69b080e