The current PHP ssh2 extension via PECL won't compile under PHP7.
Using a more recent version via PHP's GitHub we can make this work.
$ sudo apt-get install autoconf libssh2-1-dev
$ curl -LO https://github.com/php/pecl-networking-ssh2/archive/master.zip
$ unzip master.zip
$ cd pecl-networking-ssh2-master