Make sure that textpb files are treated as text

Gerrit does not recognize texpb files as text, a gitattribute
file should take care of this.
Per recommendation from
https://bugs.chromium.org/p/gerrit/issues/detail?id=15586

Test: Manual
Change-Id: Iedf4829e34442294f228fa085570bbdd1e84087d
1 file changed