alarmtimer: Fix posix-timer constification fallout

Some freezer related variables are only used when either CONFIG_POSIX_TIMER
or CONFIG_RTC_CLASS are enabled. Hide them when both are off.

Fixes: d3ba5a9a345b ("posix-timers: Make posix_clocks immutable")
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Helwig <hch@lst.de>