Add mips64 mips64le build tags (#47)

Signed-off-by: Guillaume J. Charmes <gcharmes@leaf.ag>
diff --git a/ztypes_mipsx.go b/ztypes_mipsx.go
index 425afc8..f0ce740 100644
--- a/ztypes_mipsx.go
+++ b/ztypes_mipsx.go
@@ -2,7 +2,7 @@
 // cgo -godefs types.go
 
 // +build linux
-// +build mips mipsle
+// +build mips mipsle mips64 mips64le
 
 package pty