Add PAGE_SIZE constant

src/hulk/hippo.d

@@ -8,6 +8,9 @@ import hulk.bootinfo;
 import hulk.klog;
 import hulk.linker_addresses;
 
+/** Page size. */
+enum size_t PAGE_SIZE = 4096u;
+
 struct hippo
 {
     /**
@@ -80,7 +83,7 @@ struct hippo
                     {
                         free_page(phys);
                     }
-                    phys += 4096u;
+                    phys += PAGE_SIZE;
                 }
             }
         }