Fix zend_extension directive by whether php 5.2 or not

Fix 'zend_extension'  directive in xdebug.ini by whether you configure php 5.2 or not.
__
php5.2 (5.2.17 of php-build is Thread Safe version.)
```
zend_extension_ts=".../xdebug.so"
```
other (as it is)
```
zend_extension=".../xdebug.so"
```