fix some middle-of-C-block variable declrations

src/racket/gc/mark_rts.c

@@ -369,8 +369,10 @@ ptr_t GC_approx_sp(void)
 #   ifdef _MSC_VER
 #     pragma warning(disable:4172)
 #   endif
+    {
       ptr_t addr = ((ptr_t)(&dummy));
       return addr;
+    }
 #   ifdef _MSC_VER
 #     pragma warning(default:4172)
 #   endif

src/racket/gc/new_hblk.c

@@ -164,8 +164,10 @@ ptr_t GC_build_fl(struct hblk *h, size_t sz, GC_bool clear, ptr_t list)
    * put p (which is now head of list of objects in *h) as first
    * pointer in the appropriate free list for this size.
    */
+  {
     word *tmp = (word *)h->hb_body;
     obj_link(tmp) = list;
+  }
   return ((ptr_t)p);
 }