vm.c: always export rb_frame_method_id_and_class

Functions declared in include/ruby/backward.h is exported only when
the condition `!defined RUBY_EXPORT && !defined RUBY_NO_OLD_COMPATIBILITY`
is met (i.e. included by include/ruby/ruby.h).

So if it is not the case when ruby is built, this will not be exported.
This was not intentional at r60994.

[Bug #14192]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61296 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm.c

@@ -2049,7 +2049,7 @@ rb_ec_frame_method_id_and_class(const rb_execution_context_t *ec, ID *idp, ID *c
     return rb_vm_control_frame_id_and_class(ec->cfp, idp, called_idp, klassp);
 }
 
-int
+RUBY_FUNC_EXPORTED int
 rb_frame_method_id_and_class(ID *idp, VALUE *klassp)
 {
     return rb_ec_frame_method_id_and_class(GET_EC(), idp, 0, klassp);