btrfs-progs: ioctl: fix build failure if BUILD_ASSERT is not defined

btrfs-progs: ioctl: fix build failure if BUILD_ASSERT is not defined

The build fails if ioctl.h is included without kerncompat.h which
defines BUILD_ASSERT, as reported. The proposed patches add kerncompat.h
but I don't want to pull that header via ioctl.h, so the fix will
provide an empty macro instead.

Reported-by: Eric Sandeen <sandeen@redhat.com>
Reported-by: Sergei Trofimovich <slyfox@gentoo.org>
Reported-by: Mikhail Pukhlikov <cynede@gentoo.org>
Signed-off-by: David Sterba <dsterba@suse.com>