hb-ot-shape-complex-arabic.cc: Fix build on Visual Studio

Visual Studio does not like declaring a enum variable within a for
statement, so fix the build by declaring the enum before doing the for
loop.

src/hb-ot-shape-complex-arabic.cc

@@ -469,8 +469,9 @@ apply_stch (const hb_ot_shape_plan_t *plan,
   DEBUG_MSG (ARABIC, NULL, "overlap for stretching is %d", overlap);
   int sign = font->x_scale < 0 ? -1 : +1;
   unsigned int extra_glyphs_needed = 0; // Set during MEASURE, used during CUT
+  typedef enum { MEASURE, CUT } step_t;
 
-  for (enum step_t { MEASURE, CUT } step = MEASURE; step <= CUT; step = (step_t) (step + 1))
+  for (step_t step = MEASURE; step <= CUT; step = (step_t) (step + 1))
   {
     unsigned int count = buffer->len;
     hb_glyph_info_t *info = buffer->info;