ktime: export ktime_divns

ktime: export ktime_divns

ktime_divns was exported in upstream as a side-effect of commit
166afb64511eef08e13331b970c44fe91cea45ef (ktime: Sanitize
ktime_to_us/ms conversion). But we do not want the commit given ktime
is not nanoseconds in 3.12 yet.

So we only export the function here as it is needed by upstream commit
d2c5cf88d5282de258f4eb6ab40040b80a075cd8 (ALSA: hrtimer: Handle
start/stop more properly):
ERROR: "ktime_divns" [sound/core/snd-hrtimer.ko] undefined!

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>