fix(candle-kernels): conditionally link stdc++ for non-MSVC targets

On Windows with MSVC, linking against stdc++ fails because it is a
GNU/GCC library that does not exist in the MSVC toolchain. MSVC uses
its own C++ runtime which is linked automatically.

This fix wraps the stdc++ link directive in a target check, matching
the existing pattern in candle-flash-attn/build.rs.

Fixes #3277

Author: Elvis339

candle-kernels/build.rs

@@ -46,7 +46,9 @@ fn main() {
     println!("cargo:rustc-link-search={}", out_dir.display());
     println!("cargo:rustc-link-lib=moe");
     println!("cargo:rustc-link-lib=dylib=cudart");
-    println!("cargo:rustc-link-lib=stdc++");
+    if !is_target_msvc {
+        println!("cargo:rustc-link-lib=stdc++");
+    }
 }
 
 fn remove_lines<P: AsRef<std::path::Path>>(file: P, patterns: &[&str]) {