__cplusplus guard fixed

This error prevented using this header in cpp code directly.

Author: byq77

targets/TARGET_STM/TARGET_STM32F4/pwmout_device.h

@@ -54,4 +54,8 @@ extern const pwm_apb_map_t pwm_apb_map_table[];
 
 #endif // DEVICE_PWMOUT
 
+#ifdef __cplusplus
+}
+#endif
+    
 #endif