GH-48787: [C++] Disable -Werror for s2n-tls (#48791)

### Rationale for this change

Fixes the test-ubuntu-24.04-cpp-thread-sanitizer failure where s2n-tls builds were failing due to strict compiler warnings (documentation and integer precision) on newer Clang versions. 

### What changes are included in this PR?

1. Added -Wno-error specifically for the s2n target in cpp/cmake_modules/ThirdpartyToolchain.cmake. This allows the build to proceed despite intentional warnings in the s2n-tls project that Clang 18 now flags as errors.

### Are these changes tested?

I am relying on the GitHub CI to verify that this suppression resolves the Ubuntu 24.04 compiler failures.

### Are there any user-facing changes?

No. These changes only affect the C++ build system and third-party dependency management.

* GitHub Issue: #48787

Authored-by: Meher <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: meher745

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3984,6 +3984,11 @@ function(build_awssdk)
         # We don't need to link aws-lc. It's used only by s2n-tls.
       elseif("${AWSSDK_PRODUCT}" STREQUAL "s2n-tls")
         list(PREPEND AWSSDK_LINK_LIBRARIES s2n)
+        # Disable -Werror for s2n-tls: it has Clang 18 warnings that it intentionally allows.
+        # See: https://github.com/aws/s2n-tls/issues/5696
+        if(TARGET s2n)
+          target_compile_options(s2n PRIVATE -Wno-error)
+        endif()
       else()
         list(PREPEND AWSSDK_LINK_LIBRARIES ${AWSSDK_PRODUCT})
         # This is for find_package(aws-*) in aws-crt-cpp and aws-sdk-cpp.