perf_counter: Fix userspace build

recent userspace (F11) seems to already include the
linux/unistd.h bits which means we cannot include the version
in the kernel sources due to the header guards being the same.

Ensure we include the kernel version first.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <20090523163012.739756497@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>