Added yield() declaration when compiling for MONGOOSE_OS, which is added do mos.yml as well.

mos.yml

@@ -15,6 +15,7 @@ cdefs:
   ARDUINO: 150
   RH_PLATFORM: RH_PLATFORM_ESP8266
   TEENSYDUINO: 0
+  MONGOOSE_OS: 1
 
 tags:
   - arduino

src/RadioHead.h

@@ -875,6 +875,9 @@
  #define YIELD yield();
 #elif (RH_PLATFORM == RH_PLATFORM_ESP8266)
 // ESP8266 also hash it
+#if defined(MONGOOSE_OS)
+extern void yield(void);
+#endif
  #define YIELD yield();
 #else
  #define YIELD