Fix requested for C++ compatibility

I'd like to thank whoever fixed Zend/zend_interfaces.h in RC2 so
that it avoids a "duplicate symbol" link error when building PHP with
a C++ compiler. However, a similar issue was introduced in RC2 in
Zend/zend_exceptions.h:

ZEND_API void (*zend_throw_exception_hook)(zval *ex TSRMLS_DC);

It would be great if someone could prepend "extern" to the above line.

Thanks,

• Mark