Avoid GCC 7 compilation warning in eval.c

* src/eval.c (push_handler_nosignal): Use CACHEABLE to work around
GCC compilation warning.  Suggested by Paul Eggert <eggert@cs.ucla.edu>
in http://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00492.html.

src/eval.c

@@ -1428,7 +1428,7 @@ push_handler (Lisp_Object tag_ch_val, enum handlertype handlertype)
 struct handler *
 push_handler_nosignal (Lisp_Object tag_ch_val, enum handlertype handlertype)
 {
-  struct handler *c = handlerlist->nextfree;
+  struct handler *CACHEABLE c = handlerlist->nextfree;
   if (!c)
     {
       c = malloc (sizeof *c);