Add missing `gf180mcu_fd_bd_sram` to .gitmodules

Fixes #13.

```
git submodule update --init --recursive
fatal: No url found for submodule path 'libraries/gf180mcu_fd_bd_sram/latest' in .gitmodules
```

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>