selftests: exec: update gitignore for load_address

The name of the "load_address" objects has been modified, but the
corresponding entry in the gitignore file must be updated.

Update the load_address entry in the gitignore file to account for
the new names, adding an exception to keep on tracking load_address.c.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>