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>

include/qemu/compiler.h

@@ -33,7 +33,7 @@
 #define cat(x,y) x ## y
 #define cat2(x,y) cat(x,y)
 #define QEMU_BUILD_BUG_ON(x) \
-    typedef char cat2(qemu_build_bug_on__,__LINE__)[(x)?-1:1];
+    typedef char cat2(qemu_build_bug_on__,__LINE__)[(x)?-1:1] __attribute__((unused));
 
 #if defined __GNUC__
 # if !QEMU_GNUC_PREREQ(4, 4)