Bug 1453551 - Don't use i8 as c_char in xpcom-gtest. r=mystor

c_char isn't i8 on some platforms such as Android/arm, so we should use c_char
instead.

MozReview-Commit-ID: 486BCGvYAE0

--HG--
extra : rebase_source : 0ee9d510de5cd8ab20daa1709c7425ca95dc8793

xpcom/rust/gtest/xpcom/test.rs

@@ -9,6 +9,7 @@ extern crate xpcom;
 
 extern crate nserror;
 
+use std::os::raw::c_char;
 use std::ptr;
 use std::ffi::{CStr, CString};
 use xpcom::interfaces;
@@ -29,7 +30,7 @@ pub unsafe extern fn Rust_ObserveFromRust() -> *const interfaces::nsIObserverSer
         unsafe fn Observe(
             &self,
             _subject: *const interfaces::nsISupports,
-            topic: *const i8,
+            topic: *const c_char,
             _data: *const i16
         ) -> nsresult {
             *self.run = true;