Use namespace for OpenType tables

Avoids USHORT, SHORT, ULONG, LONG clashes with Windows API.

src/hb-ot-hmtx-table.hh

diff --git a/src/hb-ot-hmtx-table.hh b/src/hb-ot-hmtx-table.hh
index cc7cfbb..feb6016 100644
--- a/src/hb-ot-hmtx-table.hh
+++ b/src/hb-ot-hmtx-table.hh
@@ -30,6 +30,8 @@
 #include "hb-open-type-private.hh"
 
 
+namespace OT {
+
 
 /*
  * hmtx -- The Horizontal Metrics Table
@@ -83,4 +85,8 @@
   DEFINE_SIZE_ARRAY2 (0, longHorMetric, leftSideBearingX);
 };
 
+
+} // namespace OT
+
+
 #endif /* HB_OT_HMTX_TABLE_HH */