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>
diff --git a/ioctl.c b/ioctl.c
index e9a503d..3f6c410 100644
--- a/ioctl.c
+++ b/ioctl.c
@@ -86,8 +86,10 @@
 	case 0x03:
 	case 0x12:
 		return block_ioctl(tcp, code, arg);
+#ifdef HAVE_SCSI_SG_H
 	case 0x22:
 		return scsi_ioctl(tcp, code, arg);
+#endif
 	case 'L':
 		return loop_ioctl(tcp, code, arg);
 	case 'M':