Fix us ticker build error

Author: multiplemonomials

platform/include/platform/mbed_wait_api.h

@@ -154,10 +154,18 @@ inline void _wait_us_inline(unsigned int us)
 #include <chrono>
 
 // Override of wait_us() allowing a std::chrono type convertible to microseconds to be passed in.
+#if defined US_TICKER_PERIOD_NUM
 static inline void _wait_us_inline(std::chrono::microseconds const us)
 {
     _wait_us_inline(us.count());
 }
+#else
+static inline void wait_us(std::chrono::microseconds const us)
+{
+    wait_us(us.count());
+}
+#endif
+
 #endif
 
 #endif