php: pdo mysql extension needs hash as well

Fixes:
http://autobuild.buildroot.net/results/69cf9326539c8df8fa50c5e7acb2ce3bb985ede2/



The PDO mysql extension also needs the hash extension - so select it,
similar to how it was done for the mysqli extension in commit 65f96452
(php: fix build with mysqli).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>