Remove unnecessary files in release tarball

This patch adds a few entries in .gitattributes to specify files that
should never end up in a distribution tarball.

Signed-off-by: Hu Keping <hukeping@huawei.com>

.gitattributes

@@ -2,3 +2,9 @@
 *.txt     text
 *.c       text
 *.h       text
+
+# For git archive
+.gitignore              export-ignore
+.gitattributes          export-ignore
+.travis.yml             export-ignore
+.vscode                 export-ignore