Revert "Stop using std::ffi::c_char as it doesn't exist on Android."

This reverts commit ea2d53ffddf29897f78b946caf8d4bab389958ee.

Reason for revert: Rust was rolled and should have c_char on Android now.

Original change's description:
> Stop using std::ffi::c_char as it doesn't exist on Android.
>
> R=​[email protected]
>
> Bug: 1381552
> Change-Id: I10bb95d4695cb3331fd2dc7b496c088a4e7259a2
> Cq-Include-Trybots: luci.chromium.try:linux-rust-x64-dbg,linux-rust-x64-rel
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4007075
> Commit-Queue: Łukasz Anforowicz <[email protected]>
> Reviewed-by: Łukasz Anforowicz <[email protected]>
> Auto-Submit: danakj <[email protected]>
> Commit-Queue: danakj <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1067731}

Bug: 1381552
Change-Id: Ie50053f9164d24fadc6b167c189b4ac0b9beae08
Cq-Include-Trybots: luci.chromium.try:android-rust-arm-dbg,linux-rust-x64-dbg,linux-rust-x64-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4018495
Reviewed-by: Łukasz Anforowicz <[email protected]>
Commit-Queue: danakj <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1069421}
diff --git a/testing/rust_gtest_interop/rust_gtest_interop.rs b/testing/rust_gtest_interop/rust_gtest_interop.rs
index 4399aa6..44efe95 100644
--- a/testing/rust_gtest_interop/rust_gtest_interop.rs
+++ b/testing/rust_gtest_interop/rust_gtest_interop.rs
@@ -125,18 +125,18 @@
         extern "C" {
             // The C++ mangled name for rust_gtest_interop::rust_gtest_add_failure_at(). This comes
             // from `objdump -t` on the C++ object file.
-            fn _ZN18rust_gtest_interop25rust_gtest_add_failure_atEPKhiS1_(
-                file: *const u8,
+            fn _ZN18rust_gtest_interop25rust_gtest_add_failure_atEPKciS1_(
+                file: *const std::ffi::c_char,
                 line: i32,
-                message: *const u8,
+                message: *const std::ffi::c_char,
             );
 
         }
         unsafe {
-            _ZN18rust_gtest_interop25rust_gtest_add_failure_atEPKhiS1_(
-                null_term_file.as_ptr() as *const u8,
+            _ZN18rust_gtest_interop25rust_gtest_add_failure_atEPKciS1_(
+                null_term_file.as_ptr(),
                 line.try_into().unwrap_or(-1),
-                null_term_message.as_ptr() as *const u8,
+                null_term_message.as_ptr(),
             )
         }
     }