Moved ZEND_DECLARE_MODULE_GLOBALS out of the header file. (PHP extension)

Hi,

I have moved ZEND_DECLARE_MODULE_GLOBALS into the msgpack.c file and added ZEND_EXTERN_MODULE_GLOBALS in source files that reference global variables.

This fixes linker error I received when compiling PHP module:

``` 
ld: duplicate symbol _msgpack_globals in .libs/msgpack_pack.o and .libs/msgpack.o 
```