x86/tsc: Add missing header to tsc_msr.c

Add a missing header otherwise compiler warns about missed prototype:

CC      arch/x86/kernel/tsc_msr.o
arch/x86/kernel/tsc_msr.c:73:15: warning: no previous prototype for ‘cpu_khz_from_msr’ [-Wmissing-prototypes]
   unsigned long cpu_khz_from_msr(void)
                 ^~~~~~~~~~~~~~~~

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Link: https://lkml.kernel.org/r/20180629193113.84425-4-andriy.shevchenko@linux.intel.com