posix: shm: fix build error with minimal libc

In `lib/posix/options/shm.c` we don't include stdio.h
header but use standard SEEK_xxx definitions (i.e. `SEEK_SET`)
which cause build issues (if the minimal libc is used).

Fix that.

Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>