Fix build on ESP32 (caused by espressif/arduino-esp32#7941)

Author: bblanchon

examples/JsonFilterExample/JsonFilterExample.ino

@@ -15,7 +15,7 @@ void setup() {
  continue;
 
  // The huge input: an extract from OpenWeatherMap response
- const __FlashStringHelper* input_json = F(
+ auto input_json = F(
  "{\"cod\":\"200\",\"message\":0,\"list\":[{\"dt\":1581498000,\"main\":{"
  "\"temp\":3.23,\"feels_like\":-3.63,\"temp_min\":3.23,\"temp_max\":4.62,"
  "\"pressure\":1014,\"sea_level\":1014,\"grnd_level\":1010,\"humidity\":"