Move FreeListBlock internal into source

freelist/freelist.c

@@ -6,6 +6,10 @@
 #include <stdlib.h>
 #include <string.h>
 
+struct FreeListBlock {
+    struct FreeListBlock *next;
+};
+
 /* Initialize the FreeList */
 int fl_init(FreeList *fl, uintptr_t start, uintptr_t end, size_t itemsize) {
     size_t size = ALIGN(itemsize);