Fix pointer sign warnings in auth.h

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>
diff --git a/tirpc/rpc/auth.h b/tirpc/rpc/auth.h
index a5e4228..b6a49d4 100644
--- a/tirpc/rpc/auth.h
+++ b/tirpc/rpc/auth.h
@@ -213,13 +213,13 @@
 static __inline int
 auth_get(AUTH *auth)
 {
-    return atomic_add_uint32_t(&auth->ah_refcnt, 1);
+    return atomic_add_int32_t(&auth->ah_refcnt, 1);
 }
 
 static __inline int
 auth_put(AUTH *auth)
 {
-    return atomic_sub_uint32_t(&auth->ah_refcnt, 1);
+    return atomic_sub_int32_t(&auth->ah_refcnt, 1);
 }