First compilation fixes, thanks to Jenkins first comments

radio/src/debug.cpp

@@ -35,6 +35,7 @@
  */
 
 #include "opentx.h"
+#include <stdarg.h>
 
 #if defined(SIMU)
 traceCallbackFunc traceCallback = 0;

radio/src/serial.cpp

@@ -36,11 +36,8 @@
 
 #include "opentx.h"
 #include "serial.h"
-
-extern "C" {
-  #include <stdio.h>
-  #include <stdarg.h>
-}
+#include <stdarg.h>
+#include <stdio.h>
 
 #define PRINTF_BUFFER_SIZE    256