unittest: Include standard header for `errno`

This fixes a build issue with the latest LLVM toolchain.

Change-Id: I7145d0d1aea6769ecab0c5286a71167fabb13d38
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minijail/+/3891224
Reviewed-by: Christian Blichmann <[email protected]>
Commit-Queue: Christian Blichmann <[email protected]>
Tested-by: Christian Blichmann <[email protected]>
Reviewed-by: Mike Frysinger <[email protected]>
diff --git a/unittest_util.h b/unittest_util.h
index 54993d7..c6cb62f 100644
--- a/unittest_util.h
+++ b/unittest_util.h
@@ -6,6 +6,7 @@
  * Utility functions for unit tests.
  */
 
+#include <errno.h>
 #include <ftw.h>
 #include <stdio.h>
 #include <stdlib.h>