Fix some imports and paths

src/libstd/sys/unix/stack_overflow.rs

@@ -39,7 +39,6 @@ mod imp {
     use libc::{sigaltstack, SIGSTKSZ, SS_DISABLE};
     use libc::{sigaction, SIGBUS, SIG_DFL,
                SA_SIGINFO, SA_ONSTACK, sighandler_t};
-    use libc;
     use libc::{mmap, munmap};
     use libc::{SIGSEGV, PROT_READ, PROT_WRITE, MAP_PRIVATE, MAP_ANON};
     use libc::MAP_FAILED;