lib/vsprintf.c: add missing EXPORT_SYMBOL(simple_strtoll)

Add a missing EXPORT_SYMBOL.

I must be the first person that wants to use this function :-)

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>