drivers: serial: b91: add missing RISC-V PLIC header

Driver was using PLIC without including necessary headers.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

drivers/serial/uart_b91.c

@@ -11,6 +11,7 @@
 #include <zephyr/drivers/uart.h>
 #include <zephyr/drivers/pinctrl.h>
 #include <zephyr/irq.h>
+#include <zephyr/drivers/interrupt_controller/riscv_plic.h>
 
 
 /* Driver dts compatibility: telink,b91_uart */