pxe: Use strlcpy() instead of strcpy() in label_boot()

The intention here is to nul-terminate the result string, so use the
correct function for that.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/pxe_utils.c b/boot/pxe_utils.c
index a92bb89..2c87aba 100644
--- a/boot/pxe_utils.c
+++ b/boot/pxe_utils.c
@@ -593,7 +593,7 @@
 		}
 
 		if (label->append)
-			strncpy(bootargs, label->append, sizeof(bootargs));
+			strlcpy(bootargs, label->append, sizeof(bootargs));
 
 		strcat(bootargs, ip_str);
 		strcat(bootargs, mac_str);