fix typo

Author: phoepsilonix

build.rs

@@ -1,6 +1,6 @@
 fn main() {
     let target_os = std::env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
-    if target_os = "windows" {
+    if target_os == "windows" {
         // https://github.com/microsoft/mimalloc/blob/af21001f7a65eafb8fb16460b018ebf9d75e2ad8/CMakeLists.txt#L487
         //let libs = ["psapi", "shell32", "user32", "advapi32", "bcrypt"];
         let libs = ["advapi32"];