elf: Fix compile error with -DNDEBUG [BZ #18755]

Compilation fails when building with -DNDEBUG after commit a3189f66.
Here is the error:

dl-close.c: In function ‘_dl_close_worker’:
dl-close.c:140:22: error: unused variable ‘nloaded’ [-Werror=unused-variable]
  140 |   const unsigned int nloaded = ns->_ns_nloaded;

Add __attribute_maybe_unused__ for‘nloaded’to fix it.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>