Nullability check for error module

Bugs: b/245972273
Test: adb shell
Change-Id: I9d1c20cf2a7e1e4201c3c54a321c9d9f896c92e2
diff --git a/libc/include/errno.h b/libc/include/errno.h
index b6a4c7e..d481cfc 100644
--- a/libc/include/errno.h
+++ b/libc/include/errno.h
@@ -49,7 +49,7 @@
  *
  * @private
  */
-int* __errno(void) __attribute_const__;
+int* _Nonnull __errno(void) __attribute_const__;
 
 /**
  * [errno(3)](http://man7.org/linux/man-pages/man3/errno.3.html) is the last error on the calling