Use .gitattributes to prevent pot header EOL issue

e.g.
Mixed EOL introduced in commit ed8c6f5c45733723b1af3d6645cb38412e963151
and it is fixed in commit 595488f712230a6921e7a48e9b338d4c30d68069

*Note* Re-checkout TortoisePotHeader.txt is needed.

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>