Use same const qualifiers for insert @ head and @ tail.

Addresses some compiler warnings about different const qualifiers.

source/src/utils/doublylinkedlist.c

@@ -49,7 +49,7 @@ void DoublyLinkedListNodeDestroy(const DoublyLinkedList *const list,
 }
 
 void DoublyLinkedListInsertAtHead(DoublyLinkedList *const list,
-                                  void *data) {
+                                  const void *const data) {
   OPENER_ASSERT(list->allocator != NULL);
   DoublyLinkedListNode *new_node = DoublyLinkedListNodeCreate(data,
                                                               list->allocator);

source/src/utils/doublylinkedlist.h

@@ -48,7 +48,7 @@ void DoublyLinkedListNodeDestroy(const DoublyLinkedList *const list,
                                  DoublyLinkedListNode **node);
 
 void DoublyLinkedListInsertAtHead(DoublyLinkedList *const list,
-                                  void *data);
+                                  const void *const data);
 
 void DoublyLinkedListInsertAtTail(DoublyLinkedList *const list,
                                   const void *const data);