Fix build error with CONFIG_SYSTEM_TIME64

sched/clock/clock.h

@@ -66,6 +66,7 @@
 /********************************************************************************
  * Global Variables
  ********************************************************************************/
+
 #if !defined(CONFIG_SCHED_TICKLESS) && !defined(__HAVE_KERNEL_GLOBALS)
   /* The system clock exists (CONFIG_SCHED_TICKLESS), but it not prototyped
    * globally in include/nuttx/clock.h.

sched/clock/clock_systimer.c

@@ -52,6 +52,7 @@
 /* See nuttx/clock.h */
 
 #undef clock_systimer
+#undef clock_systimer64
  
 /****************************************************************************
  * Private Data