Fix error with conflicting declaration of a prototype (Arduino sketch preprocessor)

tests/ino2cpp/basic/basic.ino

@@ -49,4 +49,12 @@ void fooCallback(){
 
 }
 
+extern "C" {
+void some_extern(const char *fmt, ...);
+};
+
+void some_extern(const char *fmt, ...) {
+
+}
+
 // юнікод