xtensa: fix wrong extern declaration renamed in code using it

The variable ccount_nsec has been renamed to nsec_per_ccount
in arch/xtensa/kernel/time.c in 2b8aea74

 (2007-08-05),
but the fix failed to rename the variable in
arch/xtensa/include/asm/timex.h as well.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>