lib: use const consistently for zserv path

The global variable is missing its const, but the accessor function has
a meaningless extra const in exchange...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/zclient.c b/lib/zclient.c
index 963c705..1e05d6d 100644
--- a/lib/zclient.c
+++ b/lib/zclient.c
@@ -41,7 +41,7 @@
 
 extern struct thread_master *master;
 
-char *zclient_serv_path = NULL;
+const char *zclient_serv_path = NULL;
 
 /* This file local debug flag. */
 int zclient_debug = 0;
@@ -1053,7 +1053,7 @@
     }
 }
 
-const char *const zclient_serv_path_get()
+const char *zclient_serv_path_get()
 {
   return zclient_serv_path ? zclient_serv_path : ZEBRA_SERV_PATH;
 }