Add missing #include.

--- window/wselect.c	2000-03-23 14:36:30.000000000 +0000
+++ window/wselect.c	2025-07-04 18:34:10.637507608 +0100
@@ -7,6 +7,7 @@
 #include <X11/Xutil.h>
 #include <X11/Xatom.h>
 #include <X11/cursorfont.h>
+#include <X11/Xmu/WinUtil.h>
 
 #define USED(x)
 #define MASK (ButtonPressMask|ButtonReleaseMask)