platforms: use define for lockstep scheduler

Instead of using the define __PX4_POSIX_SITL it makes more sense to have
a define just to determine if the lockstep scheduler should be used.

src/platforms/px4_time.h

@@ -9,7 +9,7 @@
 #define clockid_t int
 #endif
 
-#if defined(__PX4_POSIX_SITL) || defined(__PX4_QURT)
+#if defined(ENABLE_LOCKSTEP_SCHEDULER) || defined(__PX4_QURT)
 
 __BEGIN_DECLS
 __EXPORT int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);