build: do not gitignore "static" directories below the root

Every django app may contain a separated "static" directory.  These are
part of the build process.
Previously the gitignore entry matched any "static" directory - even in
sub directories.
Now it ignores only the "static" directory at the top level.
3 jobs for master in 3 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #5760
make_deb_package

00:02:09

 
  Test
passed #5762
deb_package_test

00:01:30

passed #5761
make_test

00:02:29