eval.c: extra modifier

* eval.c (rb_longjmp): remove an extra modifier from the forward
  declaration to match the actual definition.  [ruby-core:59451]
  [Bug #9338]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44483 b2dd03c8-39d4-4d8f-98ff-823fe69b080e