fix: add (require :sb-posix) before SIGWINCH handler

sb-posix is a built-in SBCL contrib, available via require.
Without it, sb-posix:sigwinch causes a reader error and the
eval-when for the SIGWINCH handler never executes, making
*terminal-resized-p* always nil and resize detection broken.

src/components/input.lisp

@@ -224,6 +224,7 @@ Returns a mouse-event struct."
 
 #+sbcl
 (eval-when (:load-toplevel :execute)
+  (require :sb-posix)
   (sb-sys:enable-interrupt sb-posix:sigwinch
     (lambda (signal info context)
       (declare (ignore signal info context))