Make headers in extras more cpp friendly

extras/dht/dht.h

@@ -17,6 +17,10 @@
 // Type of sensor to use
 #define DHT_TYPE    DHT22
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * Read data from sensor on specified pin.
  *
@@ -35,4 +39,8 @@ bool dht_read_data(uint8_t pin, int16_t *humidity, int16_t *temperature);
  */
 bool dht_read_float_data(uint8_t pin, float *humidity, float *temperature);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  // __DHT_H__