[subset] only preprocess in benchmark when experimental api is enabled.

perf/benchmark-subset.cc

diff --git a/perf/benchmark-subset.cc b/perf/benchmark-subset.cc
index 9bf3447..0451c11 100644
--- a/perf/benchmark-subset.cc
+++ b/perf/benchmark-subset.cc
@@ -101,9 +101,13 @@
 // the subsetting operations.
 static hb_face_t* preprocess_face(hb_face_t* face)
 {
+  #ifdef HB_EXPERIMENTAL_API
   hb_face_t* new_face = hb_subset_preprocess(face);
   hb_face_destroy(face);
   return new_face;
+  #else
+  return face;
+  #endif
 }
 
 /* benchmark for subsetting a font */