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

1 files changed, 3 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 8a5ba7db3d1..947c2827b8e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -7117,6 +7117,9 @@ AC_CACHE_CHECK([for usable FIONREAD], [emacs_cv_usable_FIONREAD],
                             #ifdef USG5_4
                             # include <sys/filio.h>
                             #endif
+                            #ifdef HAVE_UNISTD_H
+                            # include <unistd.h> /* defines ioctl() on Solaris */
+                            #endif
                           ]],
                           [[int foo = ioctl (0, FIONREAD, &foo);]])],
          [emacs_cv_usable_FIONREAD=yes],