[glibc] elf: Fix compile error with -Werror and -DNDEBUG

[Florian Weimer <fw@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=5e89ed42fd8997414732525c9460878d65363b3f

commit 5e89ed42fd8997414732525c9460878d65363b3f
Author: Yang Yanchao <yangyanchao6@huawei.com>
Date:   Fri Apr 15 17:25:05 2022 +0800

    elf: Fix compile error with -Werror and -DNDEBUG
    
    Using -Werror and -DNDEBUG at the same time will trigger the
    following compiler error:
    
    cache.c: In function 'save_cache':
    cache.c:758:15: error: unused variable 'old_offset' [-Werror=unused-variable]
      758 |       off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
          |               ^~~~~~~~~~
    
    -DNDEBUG disables the assertion, making old_offset unused.
    Use __attribute__ ((unused)) to disable this warning.

Diff:
---
 elf/cache.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/elf/cache.c b/elf/cache.c
index abe2e49a66..3d7d3a67bf 100644
--- a/elf/cache.c
+++ b/elf/cache.c
@@ -727,7 +727,8 @@ save_cache (const char *cache_name)
   if (opt_format != opt_format_old)
     {
       /* Align file position to 4.  */
-      off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
+      __attribute__ ((unused)) off64_t old_offset
+        = lseek64 (fd, extension_offset, SEEK_SET);
       assert ((unsigned long long int) (extension_offset - old_offset) < 4);
       write_extensions (fd, str_offset, extension_offset);
     }