[CI] make compiler happy

Author: pennam

extras/test/include/util/String.h

@@ -36,8 +36,9 @@ namespace arduino {
 //     -felide-constructors
 //     -std=c++0x
 
-class __FlashStringHelper;
-#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))
+#define PSTR(str) (str)
+typedef void __FlashStringHelper;
+#define F(string_literal) (reinterpret_cast<const void *>(PSTR(string_literal)))
 
 // An inherited class for holding the result of a concatenation.  These
 // result objects are assumed to be writable by subsequent concatenations.