Fix the conditional macro name [ci skip]

`RUBY_VM_CRITICAL_SECTION` is not used anywhere.

thread.c

@@ -1474,7 +1474,7 @@ static inline int
 blocking_region_begin(rb_thread_t *th, struct rb_blocking_region_buffer *region,
                       rb_unblock_function_t *ubf, void *arg, int fail_if_interrupted)
 {
-#ifdef RUBY_VM_CRITICAL_SECTION
+#ifdef RUBY_ASSERT_CRITICAL_SECTION
     VM_ASSERT(ruby_assert_critical_section_entered == 0);
 #endif
     VM_ASSERT(th == GET_THREAD());