alpha: wire up io_pgetevents system call

The io_pgetevents system call was added in linux-4.18 but has
no entry for alpha:

warning: #warning syscall io_pgetevents not implemented [-Wcpp]

Assign a the next system call number here.

Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>