mmap_unix: fix a clippy warning

warning: this import is redundant
  --> src/mmap_unix.rs:20:1
   |
20 | use libc;
   | ^^^^^^^^^ help: remove it entirely

Signed-off-by: Liu Jiang <[email protected]>

Author: jiangliu

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 85.1,
+  "coverage_score": 84.8,
   "exclude_path": "mmap_windows.rs",
   "crate_features": "backend-mmap,backend-atomic"
 }

src/mmap_unix.rs

@@ -17,8 +17,6 @@ use std::os::unix::io::AsRawFd;
 use std::ptr::null_mut;
 use std::result;
 
-use libc;
-
 use crate::guest_memory::FileOffset;
 use crate::mmap::{check_file_offset, AsSlice};
 use crate::volatile_memory::{self, compute_offset, VolatileMemory, VolatileSlice};