bind: use the prefix in the include guards

Fixes golang/go#12984.

Change-Id: I485e6071ff136eb3f475d03ebebf8d306f24c931
Reviewed-on: https://go-review.googlesource.com/16027
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/bind/genobjc.go b/bind/genobjc.go
index 39786a7..73f84d2 100644
--- a/bind/genobjc.go
+++ b/bind/genobjc.go
@@ -88,8 +88,8 @@
 	g.init()
 
 	g.Printf(objcPreamble, g.pkg.Path(), g.gobindOpts(), g.pkg.Path())
-	g.Printf("#ifndef __Go%s_H__\n", strings.Title(g.pkgName))
-	g.Printf("#define __Go%s_H__\n", strings.Title(g.pkgName))
+	g.Printf("#ifndef __%s%s_H__\n", g.prefix, strings.Title(g.pkgName))
+	g.Printf("#define __%s%s_H__\n", g.prefix, strings.Title(g.pkgName))
 	g.Printf("\n")
 	g.Printf("#include <Foundation/Foundation.h>")
 	g.Printf("\n\n")
diff --git a/bind/testdata/customprefix.objc.h.golden b/bind/testdata/customprefix.objc.h.golden
index 8997afc..5c5164a 100644
--- a/bind/testdata/customprefix.objc.h.golden
+++ b/bind/testdata/customprefix.objc.h.golden
@@ -3,8 +3,8 @@
 //
 // File is generated by gobind. Do not edit.
 
-#ifndef __GoCustomprefix_H__
-#define __GoCustomprefix_H__
+#ifndef __EXCustomprefix_H__
+#define __EXCustomprefix_H__
 
 #include <Foundation/Foundation.h>