Fix unary minus operator applied to unsigned int

Applying unary minus operator to unsigned int causes the following
warning on MSVS:

  warning C4146: unary minus operator applied to unsigned type, result still unsigned

Based on patch from Koji Ishi.

Fixes https://github.com/behdad/harfbuzz/pull/110

src/hb-ot-font.cc

diff --git a/src/hb-ot-font.cc b/src/hb-ot-font.cc
index 30dcdd4..3656a45 100644
--- a/src/hb-ot-font.cc
+++ b/src/hb-ot-font.cc
@@ -220,7 +220,7 @@
 			   void *user_data HB_UNUSED)
 {
   const hb_ot_font_t *ot_font = (const hb_ot_font_t *) font_data;
-  return font->em_scale_y (-ot_font->v_metrics.get_advance (glyph));
+  return font->em_scale_y (-(int) ot_font->v_metrics.get_advance (glyph));
 }
 
 static hb_bool_t