configure.ac: Fix FIONREAD check with gcc-14 on Solaris

configure.ac: Fix FIONREAD check with gcc-14 on Solaris

Before this fix, it would fail to build with gcc-14 with the error:
conftest.c: In function 'main':
conftest.c:265:11: error: implicit declaration of function 'ioctl'
 [-Wimplicit-function-declaration]
  265 | int foo = ioctl (0, FIONREAD, &foo);
      |           ^~~~~

Solaris documents ioctl() as being defined in <unistd.h>.

* configure.ac: Fix FIONREAD check with gcc-14 on Solaris.
Ref:
https://lists.gnu.org/r/emacs-devel/2024-10/msg00195.html

Copyright-paperwork-exempt: yes
(cherry picked from commit 5e5689a2a40cd16d7593a0c7ddea5511adc3bf22)