contint: explicitly require php5-dev

The contint box requires phpize to build extension, that is provided by
the php5-dev package which is currently installed as a dependency of
php5-pear.
To avoid potential future mistake, make sure phpize is explicitly
installed (via php5-dev).

Change-Id: I9dfb19ffac7c6bfe6efd6da9aba0cb1a0587cd95