Enable overriding kh_inline from user code

I've recently been busy with upgrading [pandas](/pydata/pandas) variant of khash to 0.2.8 and some of its changes over vanilla code could be moved to user code with minimal effort. One of them is supporting custom `kh_inline` definitions: `PANDAS_INLINE` detection macro is somewhat more complicated  than the one used in `khash.h`, this PR enables using it without modifying khash code by doing
```c
#define kh_inline PANDAS_INLINE
#include <khash.h>
```