Get rid of Werror=maybe-uninitialized in res_send.c.

The commit 446997ff introduced
this new usage of resplen. If build with gcc 9 -march>=z13 on s390x,
the following warning occurs:
res_send.c: In function ‘__res_context_send’:
res_send.c:539:6: error: ‘resplen’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  539 |   if (resplen > HFIXEDSZ)
      |      ^

Therefore this patch adds a further DIAG_IGNORE_NEEDS_COMMENT in the
same way as it was previously done for usages of resplen or n.
See commit d1bc2cbb.