class.c: make define_method_m static

The helper is used only inside class.c (mrb_mod_define_method_m,
mod_define_method, define_singleton_method) -- no need to expose it
through libmruby.a. Drops one of the non-`mrb_*` linkage symbols
flagged in #6858.

Co-authored-by: Claude <noreply@anthropic.com>

src/class.c

@@ -3966,7 +3966,7 @@ mrb_method_added(mrb_state *mrb, struct RClass *c, mrb_sym mid)
   }
 }
 
-mrb_value
+static mrb_value
 define_method_m(mrb_state *mrb, struct RClass *c, int vis)
 {
   mrb_sym mid;