fixed compile error.

```bash
# compile for android.
$ ./build_native.py
```

Error message as follow:

```
ssize_tjni/../../cocos2d/cocos/./3d/CCBundleReader.cpp:94:23: error: return type of out-of-line
      definition of 'cocos2d::BundleReader::tell' differs from that in the declaration
ssize_t BundleReader::tell()
                      ^
jni/../../cocos2d/cocos/./3d/CCBundleReader.h:90:14: note: previous declaration is here
    long int tell();
             ^
1 error generated.
```

cocos/3d/CCBundleReader.h

@@ -87,7 +87,7 @@ class BundleReader: public cocos2d::Ref
     /**
      * Returns the position of the file pointer.
      */
-    long int tell();
+    ssize_t tell();
 
     /**
      * Sets the position of the file pointer.