Minor, fix -Weverthing bot warnings (#1210)

test/api/test-buffer.c

diff --git a/test/api/test-buffer.c b/test/api/test-buffer.c
index 5c98a9a..5fba3b2 100644
--- a/test/api/test-buffer.c
+++ b/test/api/test-buffer.c
@@ -92,14 +92,14 @@
 }
 
 static void
-fixture_finish (fixture_t *fixture, gconstpointer user_data)
+fixture_finish (fixture_t *fixture, gconstpointer user_data HB_UNUSED)
 {
   hb_buffer_destroy (fixture->buffer);
 }
 
 
 static void
-test_buffer_properties (fixture_t *fixture, gconstpointer user_data)
+test_buffer_properties (fixture_t *fixture, gconstpointer user_data HB_UNUSED)
 {
   hb_buffer_t *b = fixture->buffer;
   hb_unicode_funcs_t *ufuncs;
@@ -294,7 +294,7 @@
 }
 
 static void
-test_buffer_positions (fixture_t *fixture, gconstpointer user_data)
+test_buffer_positions (fixture_t *fixture, gconstpointer user_data HB_UNUSED)
 {
   hb_buffer_t *b = fixture->buffer;
   unsigned int i, len, len2;
@@ -319,7 +319,7 @@
 }
 
 static void
-test_buffer_allocation (fixture_t *fixture, gconstpointer user_data)
+test_buffer_allocation (fixture_t *fixture, gconstpointer user_data HB_UNUSED)
 {
   hb_buffer_t *b = fixture->buffer;