build asm with werror

boot1/asm/prelude.s

@@ -3,4 +3,5 @@
 [org BOOT1_LOADPOINT]
 [bits 16]
 
-hlt
+prelude:
+  hlt

boot1/build.rs

@@ -56,7 +56,10 @@ impl Nasm {
     }
 
     let mut cmd = Command::new(&self.bin_path);
-    cmd.arg("-f").arg("bin");
+    cmd
+      .arg("-f")
+      .arg("bin")
+      .arg("-Werror");
 
     for include in includes {
       let mut buf = OsString::new();