Workaround the following issue: esp8266/Arduino#2078

Also see: https://travis-ci.org/felis/USB_Host_Shield_2.0/jobs/170350078

Author: Lauszus

settings.h

@@ -171,4 +171,10 @@ extern SPI_HandleTypeDef SPI_Handle; // Needed to be declared in your main.cpp
 #endif
 #endif
 
+// Workaround issue: https://github.com/esp8266/Arduino/issues/2078
+#ifdef ESP8266
+#undef PSTR
+#define PSTR(s) (s)
+#endif
+
 #endif /* SETTINGS_H */