Fix incorrect prototype on declaration of mrb_protect_atexit

`state.c` makes a prototype declaration for the private
`mrb_protect_atexit` which is defined in `error.c`. `error.c` defines
this function with a void return type, but `state.c` defines the
prototype with an `int` return type.

This mismatch prevents mruby from compiling on stricter compilers like
emscripten.

src/state.c

@@ -183,7 +183,7 @@ mrb_free_context(mrb_state *mrb, struct mrb_context *c)
   mrb_free(mrb, c);
 }
 
-int mrb_protect_atexit(mrb_state *mrb);
+void mrb_protect_atexit(mrb_state *mrb);
 
   MRB_API void
 mrb_close(mrb_state *mrb)