lib/vdso: Make __cvdso_clock_getres() static

Fix the following sparse warning in the generic vDSO library:

  linux/lib/vdso/gettimeofday.c:224:5: warning: symbol
  '__cvdso_clock_getres' was not declared. Should it be static?

Make it static and also mark it __maybe_unsed.

Fixes: 502a590a

 ("lib/vdso: Move fallback invocation to the callers")
Reported-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191128111719.8282-1-vincenzo.frascino@arm.com