init.c, mruby-io: undef DONE macro

Add #undef DONE after last usage to prevent macro redefinition
warnings in amalgamation builds.

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

mrbgems/mruby-io/src/mruby_io_gem.c

@@ -13,6 +13,7 @@ mrb_mruby_io_gem_init(mrb_state* mrb)
   mrb_init_file(mrb); DONE;
   mrb_init_file_test(mrb); DONE;
 }
+#undef DONE
 
 void
 mrb_mruby_io_gem_final(mrb_state* mrb)

src/init.c

@@ -47,3 +47,4 @@ mrb_init_core(mrb_state *mrb)
   mrb_init_version(mrb); DONE;
   mrb_init_mrblib(mrb); DONE;
 }
+#undef DONE