Fix redeclare block-scoped Base64 (#11677)

Issue found by https://github.com/dankogai/js-base64/issues/38#issuecomment-250999729

js-base64/js-base64.d.ts

@@ -19,34 +19,37 @@ add methods:
 - [ ] noConflict
  */
 
-interface Base64 {
-    /**
-     * .encode
-     * @param {String} string
-     * @return {String}
-     */
-    encode(base64: string): string;
+declare namespace Base64 {
+    const Base64: Base64Static
+    interface Base64Static {
+        /**
+         * .encode
+         * @param {String} string
+         * @return {String}
+         */
+        encode(base64: string): string;
 
-    /**
-     * .encodeURI
-     * @param {String} string
-     * @return {String}
-     */
-    encodeURI(base64: string): string
+        /**
+         * .encodeURI
+         * @param {String} string
+         * @return {String}
+         */
+        encodeURI(base64: string): string
 
-    /**
-     * .decode
-     * @param {String} string
-     * @return {String}
-     */
-    decode(base64: string): string
+        /**
+         * .decode
+         * @param {String} string
+         * @return {String}
+         */
+        decode(base64: string): string
 
-    /**
-     * Library version
-     */
-    VERSION:string
+        /**
+         * Library version
+         */
+        VERSION:string
+    }
 }
 
 declare module 'js-base64' {
-    const Base64: Base64
+    export = Base64
 }