perf tools: Fix build against newer glibc

Upstream glibc commit 295e904 added a definition for __attribute_const__
to cdefs.h.  This causes the following error when building perf:

util/include/linux/compiler.h:8:0: error: "__attribute_const__"
redefined [-Werror] /usr/include/sys/cdefs.h:226:0: note: this is the
location of the previous definition

Wrap __attribute_const__ in #ifndef as we do for __always_inline.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110818113720.GL2227@zod.bos.redhat.com
Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>