posix-timers: Unbreak CONFIG_POSIX_TIMERS=n build

The rework of the posix-cpu-timers patch series dropped the empty
declaration of struct cpu_timer for the CONFIG_POSIX_TIMERS=n case which
causes the build to fail:

./include/linux/posix-timers.h:218:20: error: field 'cpu' has incomplete type

Add it back.

Fixes: 60bda03 ("posix-cpu-timers: Utilize timerqueue for storage")
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/posix-timers.h

@@ -161,6 +161,7 @@ static inline void posix_cputimers_rt_watchdog(struct posix_cputimers *pct,
 	},
 #else
 struct posix_cputimers { };
+struct cpu_timer { };
 #define INIT_CPU_TIMERS(s)
 static inline void posix_cputimers_init(struct posix_cputimers *pct) { }
 static inline void posix_cputimers_group_init(struct posix_cputimers *pct,