[libglnx] errors: Mark GLNX_AUTO_PREFIX_ERROR() as used

[Colin Walters <walters src gnome org>]

commit 7d6a31fb9316ec85d462c26b84fb8fe9ca04ef2b
Author: Colin Walters <walters verbum org>
Date:   Thu Jul 20 15:09:16 2017 -0400

    errors: Mark GLNX_AUTO_PREFIX_ERROR() as used
    
    Since it's intentional we never use it, and `clang` barfs on this (rightly).

 glnx-errors.h |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/glnx-errors.h b/glnx-errors.h
index ecf7e85..07c30fe 100644
--- a/glnx-errors.h
+++ b/glnx-errors.h
@@ -112,7 +112,8 @@ glnx_cleanup_auto_prefix_error (GLnxAutoErrorPrefix *prefix)
   g_prefix_error (prefix->error, "%s: ", prefix->prefix);
 }
 G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(GLnxAutoErrorPrefix, glnx_cleanup_auto_prefix_error)
-#define GLNX_AUTO_PREFIX_ERROR(text, error) g_auto(GLnxAutoErrorPrefix) 
_GLNX_MAKE_ANONYMOUS(_glnxautoprefixerror_) = { text, error }
+#define GLNX_AUTO_PREFIX_ERROR(text, error) \
+  G_GNUC_UNUSED g_auto(GLnxAutoErrorPrefix) _GLNX_MAKE_ANONYMOUS(_glnxautoprefixerror_) = { text, error }
 
 /* Set @error using the value of `g_strerror (errno)`.
  *