fix typo in zipmap.h, _ZIMMAP_H should be _ZIPMAP_H

hello, there should be typo in zipmap header file, below:
```c
#ifndef _ZIMMAP_H // this should be _ZIPMAP_H not _ZIMMAP_H
#define _ZIPMAP_H
```
this bug is found in redis 2.2, 2.4, 2.6

thanks~