[PATCH v2 proxmox-rrd-migration-tool] use libc::c_char instead of i8 to fix mismatched type

[Luke Yasuda <jing@jing.rocks>]

Otherwise this fails on non-amd64 arches:

error[E0308]: mismatched types
   --> src/main.rs:374:17
    |
374 |     source[0] = file.0.as_ptr();
    |     ---------   ^^^^^^^^^^^^^^^ expected `*const i8`, found `*const u8`
    |     |
    |     expected due to the type of this binding
    |
    = note: expected raw pointer `*const i8`
               found raw pointer `*const u8`

error[E0308]: mismatched types
   --> src/main.rs:386:13
    |
381 |         let res = rrd_create_r2(
    |                   ------------- arguments to this function are incorrect
...
386 |             source.as_mut_ptr(),
    |             ^^^^^^^^^^^^^^^^^^^ expected `*mut *const u8`, found `*mut *const i8`
    |
    = note: expected raw pointer `*mut *const u8`
               found raw pointer `*mut *const i8`

Signed-off-by: Luke Yasuda <jing@jing.rocks>
---
Use std::ffi:c_char instead of the one from libc crate.
I don't know what I'm doing.
---
 src/main.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 799cde8..40acb08 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,5 +1,5 @@
 use std::{
-    ffi::{CStr, CString, OsString},
+    ffi::{c_char, CStr, CString, OsString},
     fs,
     io::ErrorKind,
     os::unix::{ffi::OsStrExt, fs::PermissionsExt},
@@ -370,7 +370,7 @@ fn do_rrd_migration(
         bail!("refusing to migrate metrics for {resource:?} - target already exists and 'force' not set!");
     }
 
-    let mut source: [*const i8; 2] = [std::ptr::null(); 2];
+    let mut source: [*const c_char; 2] = [std::ptr::null(); 2];
     source[0] = file.0.as_ptr();
 
     let target_path = CString::new(target_path.to_str().unwrap()).unwrap();
-- 
2.55.0