kernel: interrupts: Use the ksym macro when defining a kernel type

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

labs/kernel-serial-interrupt/kernel-serial-interrupt.tex

@@ -131,7 +131,7 @@ \section{Sleeping, waking up and communication}
 available for reading (i.e, if \code{buf_wr} is equal to
 \code{buf_rd})? We should put the process to sleep!
 
-To do so, add a \code{wait_queue_head_t} wait queue to our
+To do so, add a \ksym{wait_queue_head_t} wait queue to our
 \code{serial_dev} structure, named for example \code{wait}. In the
 \code{read()} function, keep things simple by directly using
 \kfunc{wait_event_interruptible} right from the start, to wait until