Declare iops3_st pad field as a function pointer

This shouldn't matter much on most platforms; but it's the "Correct" way
to do it; in case sizeof void (*)(void) differs from sizeof void

Change-Id: I2f688245f6cb8a25e7681c2cb0f372affe9772a0
Reviewed-on: http://review.couchbase.org/44134
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>