Avoid redefinition warnings for CLOCK_MONOTONIC (#1)

Signed-off-by: Dan Kouba <dan@atym.io>

Author: dkouba-atym

core/shared/platform/zephyr/platform_internal.h

@@ -286,7 +286,9 @@ typedef struct timespec os_timespec;
 #define CLOCK_REALTIME 1
 #endif
 
+#ifndef CLOCK_MONOTONIC
 #define CLOCK_MONOTONIC 4
+#endif
 
 static inline int
 os_sched_yield(void)