keep the cast for name_pointers

It is needed because currently it fails on Linux ARM64 with:
```
error[E0308]: mismatched types
    --> src/bcf/header.rs:120:17
     |
117  |             htslib::bcf_hdr_subset(
     |             ---------------------- arguments to this function are incorrect
...
120  |                 name_pointers.as_ptr(),
     |                 ^^^^^^^^^^^^^^^^^^^^^^ expected `*const *mut u8`, found `*const *mut i8`
     |
     = note: expected raw pointer `*const *mut u8`
                found raw pointer `*const *mut i8`
```

Signed-off-by: Martin Tzvetanov Grigorov <mgrigorov@apache.org>

src/bcf/header.rs

@@ -113,11 +113,13 @@ impl Header {
             .map(|&s| ffi::CString::new(s).unwrap())
             .collect();
         let name_pointers: Vec<_> = names.iter().map(|s| s.as_ptr() as *mut i8).collect();
+        #[allow(clippy::unnecessary_cast)] 
+        let name_pointers_ptr = name_pointers.as_ptr() as *const *mut c_char;
         let inner = unsafe {
             htslib::bcf_hdr_subset(
                 header.inner,
                 samples.len() as i32,
-                name_pointers.as_ptr(),
+                name_pointers_ptr,
                 imap.as_mut_ptr(),
             )
         };