Removed "rpcsvc/rex.h" from build.rs for OpenBSD

```
export TARGET=x86_64-unknown-openbsd
export QEMU=openbsd.qcow2
sh ci/run-docker.sh $TARGET
```
Passes when run localy.

Author: berkowski

libc-test/build.rs

@@ -205,7 +205,6 @@ fn main() {
 
     if openbsd {
         cfg.header("ufs/ufs/quota.h");
-        cfg.header("rpcsvc/rex.h");
         cfg.header("pthread_np.h");
         cfg.header("sys/syscall.h");
     }