Do not compile scsi ioctl decoding if <scsi/sg.h> is not available

Add a generic test for <scsi/sg.h> availability to fix build with
Bionic libc that does not provide <scsi/sg.h>.

* configure.ac (AC_CHECK_HEADERS): Add scsi/sg.h.
* ioctl.c (ioctl_decode): Do not call scsi_ioctl if !HAVE_SCSI_SG_H.
* scsi.c: Do not compile scsi ioctl decoding if !HAVE_SCSI_SG_H.

Reported-by: Elliott Hughes <enh@google.com>
3 files changed