databases/pecl-mongodb: update 1.12.0 -> 1.15.1, unbreak build

Relnotes:
https://github.com/mongodb/mongo-php-driver/releases/tag/1.12.1
https://github.com/mongodb/mongo-php-driver/releases/tag/1.13.0
https://github.com/mongodb/mongo-php-driver/releases/tag/1.14.0
https://github.com/mongodb/mongo-php-driver/releases/tag/1.14.1
https://github.com/mongodb/mongo-php-driver/releases/tag/1.14.2
https://github.com/mongodb/mongo-php-driver/releases/tag/1.15.0
https://github.com/mongodb/mongo-php-driver/releases/tag/1.15.1