cargo2android: Fix edge case in --add_workspace feature

If the last line of a Cargo.toml was a `[*]`, then appending
`[workspace]` without newlines would result in an invalid toml file.

external/crosvm/tracing/Cargo.toml is an example.

Test: ran on crosvm
Change-Id: Ib949e7f92d0205b1bc6433d1e231924832ddb536
1 file changed