Nullability check for utsname module

Bugs: b/245972273
Test: adb shell
Change-Id: I713c54998ab32e3dc9a6d53fd64523f5fdcbf79a
diff --git a/libc/include/sys/utsname.h b/libc/include/sys/utsname.h
index 1fa3187..aa8c1a0 100644
--- a/libc/include/sys/utsname.h
+++ b/libc/include/sys/utsname.h
@@ -62,6 +62,6 @@
  *
  * Returns 0 on success, and returns -1 and sets `errno` on failure.
  */
-int uname(struct utsname* __buf);
+int uname(struct utsname* _Nonnull __buf);
 
 __END_DECLS