Move location of HB_UNUSED to make MSVC happy

src/hb-subset.cc

@@ -47,9 +47,9 @@
 #include "hb-ot-layout-gpos-table.hh"
 
 
-static HB_UNUSED unsigned int
+static unsigned int
 _plan_estimate_subset_table_size (hb_subset_plan_t *plan,
-				  unsigned int table_len)
+				  unsigned int table_len) HB_UNUSED
 {
   unsigned int src_glyphs = plan->source->get_num_glyphs ();
   unsigned int dst_glyphs = plan->glyphset ()->get_population ();