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.

1 files changed, 2 insertions, 1 deletions

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);
     }