perf tests: Aename open*.c to openat*.c

perf tests: Aename open*.c to openat*.c

Since the test being tested is now openat rather than open, rename the
files to make it explicit. The patch is separeted from the first to make
it simpler to deal with any potential conflicts in the Makefile

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1429192375-13706-3-git-send-email-riku.voipio@linaro.org
[ Fixed it up wrt Build files ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>