UPSTREAM: 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: 502a590a170b ("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


(cherry picked from commit ffd08731b2d632459428612431060cf902324a8d)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I9842a0811350fc325315b25ba3f41e4b626a97c0