Fix mark_free() to work on multiple platforms

src/block_header.c

@@ -78,7 +78,7 @@ void mark_used(allocator_t *allocator, void *ptr) {
 void mark_free(allocator_t *allocator, void *ptr) {
     block_header_t *next_block_header;
     
-    get_header(ptr)->size &= (~ 0x1U);
+    get_header(ptr)->size &= (~ ((size_t) 0x1));
 
     if(allocator->border_ptr != ptr) {
         next_block_header = (block_header_t *) ((char *) ptr + get_size(ptr));