Fix compiler warning about unused function.

Fix compiler warning about unused function.

When neither of those compiler macros are available, the compiler is going to spit a warning about the unused static function.

```
coders/miff.c:164:14: warning: 'AcquireCompressionMemory' defined but not used [-Wunused-function]
 static void *AcquireCompressionMemory(void *context,const size_t items,
```

Fix this by making sure the static function is only defined when it's going to be used.