xcon: include unistd.h, it's needed for select() on MacOS

sesman/tools/xcon.c

@@ -19,6 +19,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #include <sys/types.h>
 #include <X11/Xlib.h>