Use namespace for OpenType tables

Avoids USHORT, SHORT, ULONG, LONG clashes with Windows API.
diff --git a/src/main.cc b/src/main.cc
index 07d3d69..b362744 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -36,6 +36,8 @@
 #include <stdio.h>
 
 
+using namespace OT;
+
 
 int
 main (int argc, char **argv)