posix-timers: Move posix-timer internals to core

None of these declarations is required outside of kernel/time. Move them to
an internal header.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20170530211656.394803853@linutronix.de

kernel/time/posix-clock.c

@@ -25,6 +25,8 @@
 #include <linux/syscalls.h>
 #include <linux/uaccess.h>
 
+#include "posix-timers.h"
+
 static void delete_clock(struct kref *kref);
 
 /*