Fix clang -Wvarargs compile errors

error: passing an object that undergoes default argument promotion to
'va_start' has undefined behavior [-Werror,-Wvarargs]

Change-Id: Id342beea916ec73e29e399087532caecfa19055f
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/bier/bier_test.c b/src/vnet/bier/bier_test.c
index 2c13abc..08a8c55 100644
--- a/src/vnet/bier/bier_test.c
+++ b/src/vnet/bier/bier_test.c
@@ -158,7 +158,7 @@
 
 static int
 bier_test_validate_entry (index_t bei,
-                          u16 n_buckets,
+                          int n_buckets,
                           ...)
 {
     dpo_id_t dpo = DPO_INVALID;