Make headers in extras more cpp friendly

extras/pwm/pwm.h

@@ -5,10 +5,17 @@
  * Copyright (C) 2015 Javier Cardona (https://github.com/jcard0na)
  * BSD Licensed as described in the file LICENSE
  */
+#ifndef EXTRAS_PWM_H_
+#define EXTRAS_PWM_H_
+
 #include <stdint.h>
 
 #define MAX_PWM_PINS    8
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void pwm_init(uint8_t npins, uint8_t* pins);
 void pwm_set_freq(uint16_t freq);
 void pwm_set_duty(uint16_t duty);
@@ -16,3 +23,9 @@ void pwm_set_duty(uint16_t duty);
 void pwm_restart();
 void pwm_start();
 void pwm_stop();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* EXTRAS_PWM_H_ */