mgmt: mcumgr: smp_dummy: Add missing conditional end bracket

Compiling smp_dummy.h for C++ is currently
not possible due to a missing end curly bracket.

This commit add the missing end bracket on the same
condition as the start bracket is included.

Signed-off-by: Casper Jørgensen <casperej@gmail.com>

include/zephyr/mgmt/mcumgr/transport/smp_dummy.h

@@ -98,4 +98,8 @@ void smp_dummy_disable(void);
  */
 bool smp_dummy_get_status(void);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* ZEPHYR_INCLUDE_MGMT_MCUMGR_TRANSPORT_DUMMY_H_ */