tangogps: fix doc instalation

* incorectly installs doc to /usr/doc and isn't picked by FILES_
tangogps-0.99.4: tangogps: Files/directories were installed but not shipped
  /usr/doc
  /usr/doc/tangogps
  /usr/doc/tangogps/INSTALL
  /usr/doc/tangogps/NEWS
  /usr/doc/tangogps/TODO
  /usr/doc/tangogps/README
  /usr/doc/tangogps/ChangeLog
  /usr/doc/tangogps/AUTHORS
  /usr/doc/tangogps/COPYING

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>