Demotion mrb_f_raise() in kernel.c from MRB_API

The prototype declaration of mrb_f_raise() has arleady demoted from
MRB_API in a below commit.

https://github.com/mruby/mruby/commit/0898ce982eea706c9a7b814280cdda09e82abe17

However, the entity of mrb_f_raise() in kernel.c is set MRB_API yet.
Therefore, mruby occures build error with MSVC(in Visual Studio 2022) as below.

https://github.com/groonga/groonga/actions/runs/6310140553/job/17131485675#step:11:2067

The cause of the above error that we define both mrb_f_raise() with
MRB_API and mrb_f_raise() without MRB_API.

src/kernel.c

@@ -411,7 +411,7 @@ mrb_false(mrb_state *mrb, mrb_value self)
  *     raise "Failed to create socket"
  *     raise ArgumentError, "No parameters", caller
  */
-MRB_API mrb_value
+mrb_value
 mrb_f_raise(mrb_state *mrb, mrb_value self)
 {
   mrb_value exc, mesg;