.gitmodules: ignore untracked files in brotli

.gitmodules: ignore untracked files in brotli

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2692
BrotliCompress submodule change for BaseTools causes untracked
files in BaseTools after building. This is regression for git.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>