freemodbus: add extern c block into common header for cpp

* Original commit: espressif/esp-idf@b1fe4c4cc6

components/freemodbus/common/include/esp_modbus_common.h

@@ -18,6 +18,10 @@
 
 #include "driver/uart.h"                    // for UART types
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define MB_CONTROLLER_STACK_SIZE            (CONFIG_FMB_CONTROLLER_STACK_SIZE)   // Stack size for Modbus controller
 #define MB_CONTROLLER_PRIORITY              (CONFIG_FMB_SERIAL_TASK_PRIO - 1)    // priority of MB controller task
 
@@ -135,4 +139,8 @@ typedef esp_err_t (*iface_destroy)(void);                 /*!< Interface method
 typedef esp_err_t (*iface_setup)(void*);                  /*!< Interface method setup */
 typedef esp_err_t (*iface_start)(void);                   /*!< Interface method start */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // _MB_IFACE_COMMON_H

components/freemodbus/common/include/esp_modbus_master.h

@@ -21,6 +21,10 @@
 #include "soc/soc.h"                // for BITN definitions
 #include "esp_modbus_common.h"      // for common types
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*!
  * \brief Modbus descriptor table parameter type defines.
  */
@@ -238,4 +242,8 @@ esp_err_t mbc_master_get_parameter(uint16_t cid, char* name, uint8_t* value, uin
 */
 esp_err_t mbc_master_set_parameter(uint16_t cid, char* name, uint8_t* value, uint8_t *type);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // _ESP_MB_MASTER_INTERFACE_H

components/freemodbus/common/include/esp_modbus_slave.h

@@ -24,6 +24,10 @@
 #include "freertos/event_groups.h"  // for event groups
 #include "esp_modbus_common.h"      // for common types
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @brief Parameter access event information type
  */
@@ -120,4 +124,8 @@ esp_err_t mbc_slave_get_param_info(mb_param_info_t* reg_info, uint32_t timeout);
  */
 esp_err_t mbc_slave_set_descriptor(mb_register_area_descriptor_t descr_data);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif

components/freemodbus/common/include/mbcontroller.h

@@ -13,7 +13,7 @@
  * limitations under the License.
 */
 //  mbcontroller.h
-//  mbcontroller common header
+//  mbcontroller - common Modbus controller header file
 
 #ifndef _MODBUS_CONTROLLER_COMMON
 #define _MODBUS_CONTROLLER_COMMON
@@ -28,5 +28,6 @@
 
 #include "esp_modbus_master.h"
 #include "esp_modbus_slave.h"
+
 #endif