Add libc to Rust allowlist.

Bug: 182498247
Test: Build
Change-Id: I3049967e9736e1c0c802f7678f5cafeb6644d4aa

rust/config/allowed_list.go

diff --git a/rust/config/allowed_list.go b/rust/config/allowed_list.go
index 31ec3f1..f568f27 100644
--- a/rust/config/allowed_list.go
+++ b/rust/config/allowed_list.go
@@ -6,6 +6,7 @@
 	// for an example.
 	// TODO(b/160223496): enable rustfmt globally.
 	RustAllowedPaths = []string{
+		"bionic/libc",
 		"device/google/cuttlefish",
 		"external/adhd",
 		"external/crosvm",