mark_free(): cast 0x1 as unsigned to remove a warning

src/block_header.c

@@ -80,11 +80,11 @@ 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 &= (~ 0x1);
+    get_header(ptr)->size &= (~ 0x1U);
 
     if(allocator->border_ptr != ptr) {
         next_block_header = (block_header_t *) ((char *) ptr + get_size(ptr));
-        next_block_header->previous_size &= (~ 0x1);
+        next_block_header->previous_size &= (~ 0x1U);
     }    
 }