x11: fix build on arm

The c_char type, which was used under the hood is different depending
on arch, thus use it directly instead of i8.

Fixes #3735.

src/changelog/unreleased.md

@@ -39,3 +39,7 @@ The migration guide could reference other migration examples in the current
 changelog entry.
 
 ## Unreleased
+
+### Fixed
+
+- On X11, build on arm platforms.

src/platform_impl/linux/x11/ime/context.rs

@@ -158,7 +158,7 @@ struct PreeditCallbacks {
 impl PreeditCallbacks {
     pub fn new(client_data: ffi::XPointer) -> PreeditCallbacks {
         let start_callback = create_xim_callback(client_data, unsafe {
-            mem::transmute::<usize, unsafe extern "C" fn(ffi::XIM, *mut i8, *mut i8)>(
+            mem::transmute::<usize, unsafe extern "C" fn(ffi::XIM, ffi::XPointer, ffi::XPointer)>(
                 preedit_start_callback as usize,
             )
         });