compiler: fix warning with GCC 4.8.0

GCC 4.8.0 introduces a new warning:

    block/qcow2-snapshot.c: In function 'qcow2_write_snapshots’:
    block/qcow2-snapshot.c:252:18: error: typedef 'qemu_build_bug_on__253'
              locally defined but not used [-Werror=unused-local-typedefs]
         QEMU_BUILD_BUG_ON(offsetof(QCowHeader, snapshots_offset) !=
                  ^
    cc1: all warnings being treated as errors

(Caret diagnostics aren't perfect yet with macros... :)) Work around it
with __attribute__((unused)).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1364391272-1128-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 99835e00849369bab726a4dc4ceed1f6f9ed967c)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>