Explicitly specify the void type keyword for all void functions/methods. (#1042)

include/openthread-diag.h

@@ -47,7 +47,7 @@ char *diagProcessCmd(int argc, char *argv[]);
 
 char *diagProcessCmdLine(char *string);
 
-bool isDiagEnabled();
+bool isDiagEnabled(void);
 
 #ifdef __cplusplus
 }  // extern "C"

include/openthread.h

@@ -128,7 +128,7 @@ OTAPI const char *OTCALL otGetVersionString(void);
  * @sa otApiFinalize
  *
  */
-OTAPI otApiInstance *OTCALL otApiInit();
+OTAPI otApiInstance *OTCALL otApiInit(void);
 
 /**
  * This function uninitializes the OpenThread library.