gitk: Update "Language:" header

msgfmt(1) wants this header.

Signed-off-by: YOKOTA Hiroshi <yokota@netlab.cs.tsukuba.ac.jp>

Author: qykth-git

po/ja.po

@@ -13,7 +13,7 @@ msgstr ""
 "PO-Revision-Date: 2009-11-06 01:45+0900\n"
 "Last-Translator: YOKOTA Hiroshi <yokota@netlab.cs.tsukuba.ac.jp>\n"
 "Language-Team: Japanese\n"
-"Language: \n"
+"Language: ja\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"