2015/json: fix build

Quieten go get's complaints about code examples.

Change-Id: If91dbf20df93e815fabf4d90da44ca27f724c54f
Reviewed-on: https://go-review.googlesource.com/7922
Reviewed-by: Minux Ma <minux@golang.org>
diff --git a/2015/json/dates.go b/2015/json/dates.go
index 2502f4c..2bac211 100644
--- a/2015/json/dates.go
+++ b/2015/json/dates.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/roman_numerals.go b/2015/json/roman_numerals.go
index c802100..2eb9cd6 100644
--- a/2015/json/roman_numerals.go
+++ b/2015/json/roman_numerals.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/secret.go b/2015/json/secret.go
index 698bf2d..0c6c6ec 100644
--- a/2015/json/secret.go
+++ b/2015/json/secret.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler0.go b/2015/json/unmarshaler0.go
index 1efbe96..844da80 100644
--- a/2015/json/unmarshaler0.go
+++ b/2015/json/unmarshaler0.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler0bad.go b/2015/json/unmarshaler0bad.go
index 74bfdb0..0636f74 100644
--- a/2015/json/unmarshaler0bad.go
+++ b/2015/json/unmarshaler0bad.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler0map.go b/2015/json/unmarshaler0map.go
index 22e142a..69ebd4e 100644
--- a/2015/json/unmarshaler0map.go
+++ b/2015/json/unmarshaler0map.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler1.go b/2015/json/unmarshaler1.go
index 7b34681..de5c46c 100644
--- a/2015/json/unmarshaler1.go
+++ b/2015/json/unmarshaler1.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler2.go b/2015/json/unmarshaler2.go
index 141f5ba..d61b67c 100644
--- a/2015/json/unmarshaler2.go
+++ b/2015/json/unmarshaler2.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler3.go b/2015/json/unmarshaler3.go
index 49e4faa..e3f2aa5 100644
--- a/2015/json/unmarshaler3.go
+++ b/2015/json/unmarshaler3.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (
diff --git a/2015/json/unmarshaler4.go b/2015/json/unmarshaler4.go
index 1bad743..52cdddd 100644
--- a/2015/json/unmarshaler4.go
+++ b/2015/json/unmarshaler4.go
@@ -1,3 +1,5 @@
+// +build OMIT
+
 package main
 
 import (