serial/serial_dma.c: fix warnnings

serial/serial_dma.c: In function ‘uart_recvchars_done’:
serial/serial_dma.c:407:7: warning: implicit declaration of function ‘nxsig_kill’ [-Wimplicit-function-declaration]
  407 |       nxsig_kill(dev->pid, signo);
      |       ^~~~~~~~~~

Change-Id: I9e1c0341ecce3033889d11fff9ec2b9e3dfe303b

drivers/serial/serial_dma.c

@@ -42,7 +42,7 @@
 #include <sys/types.h>
 #include <stdint.h>
 #include <debug.h>
-#include <signal.h>
+#include <nuttx/signal.h>
 
 #include <nuttx/serial/serial.h>