lib9p: fix compilation errors in example server

Reviewed by:	markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D58634

contrib/lib9p/example/Makefile

@@ -5,6 +5,6 @@ MAN=
 CFLAGS=	-pthread -g -O0
 
 LDFLAGS=-L..
-LDADD=	-lsbuf -l9p -lcasper -lcap_pwd -lcap_grp
+LIBADD+=sbuf 9p casper cap_pwd cap_grp
 
 .include <bsd.prog.mk>

contrib/lib9p/example/server.c

@@ -39,8 +39,8 @@ main(int argc, char **argv)
 {
 	struct l9p_backend *fs_backend;
 	struct l9p_server *server;
-	char *host = "0.0.0.0";
-	char *port = "564";
+	const char *host = "0.0.0.0";
+	const char *port = "564";
 	char *path;
 	bool ro = false;
 	int rootfd;