Removed "termios.h" from `build.rs` openbsd case.

Adding `termios.h` behind `rpcsvc/rex.h` does not solve the #define
clash since `rex.h` also defines the `_SYS_TERMIOS_H_` include guard.
diff --git a/libc-test/build.rs b/libc-test/build.rs
index c7d7f88..fc4db1e 100644
--- a/libc-test/build.rs
+++ b/libc-test/build.rs
@@ -206,7 +206,6 @@
     if openbsd {
         cfg.header("ufs/ufs/quota.h");
         cfg.header("rpcsvc/rex.h");
-        cfg.header("termios.h");
         cfg.header("pthread_np.h");
         cfg.header("sys/syscall.h");
     }