block: Work-around a bug in libiscsi 1.9.0 when used in gnu99 mode

block: Work-around a bug in libiscsi 1.9.0 when used in gnu99 mode

The header "scsi-lowlevel.h" of libiscsi 1.9.0 contains some bad
"inline" prototype definitions which GCC refuses to compile in its
gnu99 mode:

In file included from block/iscsi.c:52:0:
/usr/include/iscsi/scsi-lowlevel.h:810:13: error: inline function
‘scsi_set_uint16’ declared but never defined [-Werror]
 inline void scsi_set_uint16(unsigned char *c, uint16_t val);
             ^
/usr/include/iscsi/scsi-lowlevel.h:809:13: error: inline function
‘scsi_set_uint32’ declared but never defined [-Werror]
 inline void scsi_set_uint32(unsigned char *c, uint32_t val);
             ^
[...]

This has been fixed by upstream libiscsi in version 1.10.0 (see
https://github.com/sahlberg/libiscsi/commit/7692027d6c11 ), but
since we still want to support 1.9.0 for CentOS 7 / RHEL7, we
have to work-around the issue by redefining the "inline" keyword
to use the old "gnu89" mode behavior via "gnu_inline" instead.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>