[PATCH] configure.ac: Fix FIONREAD check to work with gcc-14 on Solaris

[Alan Coopersmith]

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 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/configure.ac b/configure.ac
index 8a5ba7db3d1..947c2827b8e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -7117,6 +7117,9 @@ AC_DEFUN
                            #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],
-- 
2.45.2