platforms: fix clock build for macOS (yet again)

src/lib/cdev/posix/cdev_platform.cpp

@@ -379,8 +379,8 @@ extern "C" {
 
 				// Get the current time
 				struct timespec ts;
-				// px4_sem_timedwait is implemented using CLOCK_MONOTONIC,
-				// at least for lockstep, on Qurt and on Linux.
+				// Note, we can't actually use CLOCK_MONOTONIC on macOS
+				// but that's hidden and implemented in px4_clock_gettime.
 				px4_clock_gettime(CLOCK_MONOTONIC, &ts);
 
 				// Calculate an absolute time in the future

src/platforms/px4_time.h

@@ -9,10 +9,11 @@
 #define clockid_t int
 #endif
 
+__EXPORT int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);
+
 #if defined(ENABLE_LOCKSTEP_SCHEDULER) || defined(__PX4_QURT)
 
 __BEGIN_DECLS
-__EXPORT int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);
 __EXPORT int px4_clock_settime(clockid_t clk_id, const struct timespec *tp);
 
 __EXPORT int px4_usleep(useconds_t usec);
@@ -24,7 +25,6 @@ __END_DECLS
 
 #else
 
-#define px4_clock_gettime system_clock_gettime
 #define px4_clock_settime system_clock_settime
 #define px4_usleep system_usleep
 #define px4_sleep system_sleep