stress-ng: Depend on kernel headers >= 3.3

stress-ng: Depend on kernel headers >= 3.3

perf.c need PERF_COUNT_HW_REF_CPU_CYCLES generic PMU event which
is available only since 3.3 kernel version

Fixes:
  http://autobuild.buildroot.net/results/88909b118dc927f31ed5c3b5f3513734a88085a9
  http://autobuild.buildroot.net/results/dac6a85fdd721b959175caf5629b91dee717c3c6
  http://autobuild.buildroot.net/results/88909b118dc927f31ed5c3b5f3513734a88085a9

Signed-off-by: Julien Corjon <corjon.j@ecagroup.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>