perf tools: Fix build on older systems

perf tools: Fix build on older systems

Where /usr/include/linux/const.h is not present, e.g. RHEL5.

Reported-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Link: http://lkml.kernel.org/n/tip-ypcw2mu0w7dl1rrc6ncz3pee@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>