include assert.h in cstd.h

Three src files already were including it, and another was
about to be added, so include it in include/cstd.h.

src/light.c

@@ -3,7 +3,6 @@
 /* NetHack may be freely redistributed.  See license for details.       */
 
 #include "hack.h"
-#include <assert.h>
 
 /*
  * Mobile light sources.

src/mklev.c

@@ -4,7 +4,6 @@
 /* NetHack may be freely redistributed.  See license for details. */
 
 #include "hack.h"
-#include <assert.h>
 
 /* for UNIX, Rand #def'd to (long)lrand48() or (long)random() */
 /* croom->lx etc are schar (width <= int), so % arith ensures that */

src/vision.c

@@ -3,7 +3,6 @@
 /* NetHack may be freely redistributed.  See license for details.       */
 
 #include "hack.h"
-#include <assert.h>
 
 /* Circles
  * ==================================================================*/