fix(parser): functions body placebo so code compiles

src/parser/parser.h

@@ -7,6 +7,8 @@
  *
  * @return Returns the AST representation of the next command to execute.
  *         If there is no command left to execute, retuns an AST_END node.
+ *
+ * @warning NOT IMPLEMENTED
  */
 struct ast* get_ast();
 
@@ -14,6 +16,8 @@ struct ast* get_ast();
  *
  * @return Returns the AST representation of the given command string.
  *         Returns an AST_END node if the given command is empty.
+ *
+ * @warning NOT IMPLEMENTED
  */
 struct ast* get_ast_str(char* command);