perf/x86/msr: Fix the MSR driver build

The new MSR PMU driver made use of rdtsc() which does not exist (yet) in
this tree:

  arch/x86/kernel/cpu/perf_event_msr.c:91:3: error: implicit declaration of function 'rdtsc'

Use the old rdtscll() primitive for now.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>