distinguish internal/ext. builds with FIO_INTERNAL

distinguish internal/ext. builds with FIO_INTERNAL

To distinguish between internal and external includes of
fio.h and other headers FIO_INTERNAL get applied only for
the fio (and gfio) internal build itself.

This helps to prevent double-declaration issues by making
ambigous declaration with common names coditional based on
the FIO_INTERNAL define.

Signed-off-by: Daniel Gollub <daniel.gollub@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>