make: remove trailing slashes in trusty-std/rules.mk

Adding libcore-rust build libcompiler_builtints-rust with trailing
slashes creates problems in incremental Rust builds where the same
rule can fail twice and cause parallel builds to fail.

Bug: None
Test: presubmit
Change-Id: I090c6b368f1ab4f1ae37ca0d59da8b7072e4707e
1 file changed