lang/php72: Add missing header file

Add missing file zend_smart_string.h to pkg-plist.
The allows building many more of the pecl-* ports.

PR: 223766
Submitted by: Raivo Hool <raivo.hool@gmail.com>
Reported by: Raivo Hool <raivo.hool@gmail.com>, Daniel Ylitalo <daniel@blodan.se>