perf tools: Fix getrusage() related build failure on glibc trunk

On a system running glibc trunk perf doesn't build:

    CC builtin-sched.o
builtin-sched.c: In function ‘get_cpu_usage_nsec_parent’: builtin-sched.c:399:16: error: storage size of ‘ru’ isn’t known builtin-sched.c:403:2: error: implicit declaration of function ‘getrusage’ [-Werror=implicit-function-declaration]
    [...]

Fix it by including sys/resource.h.

Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20120404084527.GA294@x4
Signed-off-by: Ingo Molnar <mingo@kernel.org>