Instead, use it on the first member, since by definition, if
any member is aligned, the whole struct must be, in order to
maintain that alignment.

Fixes compilation with some finicky compilers, like a mix of libclang/msvc

Idea for fix from Måns Rullgård.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>