config: add missing CONFIG_EXT4_FS_SUBTREE in kernel config file

After applied ext4 subtree id patch, we need to update kernel config
file to enable it.  But currently there is not CONFIG_EXT4_FS_SUBTREE
entry in kernel config file.  So fix it.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Merge pull request #56 from gnehzuil/enable-ext4-subtree

config: add missing CONFIG_EXT4_FS_SUBTREE in kernel config file