Remove DLLCallback import from CallbackReference

[dyorgio]

Provide complete information about the problem

1. Version of JNA and related jars
   jna-5.6.0.jar

2. Version and vendor of the java virtual machine
   zulu8.46.0.19-ca-fx-jdk8.0.252-macosx_x64 (Azul)

3. Operating system
   macOS BigSur 11.0.1

4. System architecture (CPU type, bitness of the JVM)
   x64, x64

5. Complete description of the problem

To reduce final uber-jar, I remove some packages accourding with OS target, to Mac and Linux I removed com.sun.jna.win32.
Removing it throws java.lang.NoClassDefFoundError: com/sun/jna/win32/DLLCallback, because class was removed :).

6. Steps to reproduce
   Remove package com.sun.jna.win32 and try to use Callbacks in any other OS.

[matthiasblaesing]

Fix was merged to master.