codegen_llvm: remove unnecessary "extern C"
codegen_llvm: remove unnecessary "extern C"

Signed-off-by: David Wood <aa743a0aaec8f7d7a1f01442503957f4d7a2d634@davidtw.co>

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust