cargo2rulesmk.py: dedup lib deps

Cargo metadata output can include the same dependency
multiple times so they need to be deduplicated in the
output rules.mk.

Bug: None
Test: cd external/rust/crates/zerocopy && ./cargo2rulesmk.py
Change-Id: I226316fe59f1811cf674d598c415de67b3f75792
1 file changed