remove references to u_char type, use 'unsigned char' instead

... u_char not being defined on Nuttx.
diff --git a/src/vty/buffer.c b/src/vty/buffer.c
index e385f9f..def2f52 100644
--- a/src/vty/buffer.c
+++ b/src/vty/buffer.c
@@ -180,7 +180,7 @@
 }
 
 /* Insert character into the buffer. */
-void buffer_putc(struct buffer *b, u_char c)
+void buffer_putc(struct buffer *b, unsigned char c)
 {
 	buffer_put(b, &c, 1);
 }
diff --git a/src/vty/vty.c b/src/vty/vty.c
index 10a323f..f34e529 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -276,7 +276,7 @@
 			p = buf;
 
 		/* Pointer p must point out buffer. */
-		buffer_put(vty->obuf, (u_char *) p, len);
+		buffer_put(vty->obuf, (unsigned char *) p, len);
 
 		/* If p is not different with buf, it is allocated buffer.  */
 		if (p != buf)