timekeeping: Fix declaration of read_persistent_wall_and_boot_offset()

It is read_persistent_wall_and_boot_offset() and not
read_persistent_clock_and_boot_offset()

Fixes: 3eca9937

 ("timekeeping: Replace read_boot_clock64() with read_persistent_wall_and_boot_offset()")
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Link: https://lkml.kernel.org/r/20180903081533.34366-1-borntraeger@de.ibm.com