modem: pipelink: Add missing extern C in header

<zephyr/modem/pipelink.h> was missing
"extern C" statement.

Signed-off-by: Jeff Welder <Jeff.Welder@ellenbytech.com>

Author: jeffwelder-ellenbytech

include/zephyr/modem/pipelink.h

@@ -11,6 +11,10 @@
 #ifndef ZEPHYR_MODEM_PIPELINK_
 #define ZEPHYR_MODEM_PIPELINK_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @brief Modem pipelink
  * @defgroup modem_pipelink Modem pipelink
@@ -157,4 +161,8 @@ void modem_pipelink_notify_disconnected(struct modem_pipelink *link);
 
 /** @} */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* ZEPHYR_MODEM_PIPELINK_ */