Omit invalid waitpid flags on OpenBSD

OpenBSD doesn't have `WEXITED`, `WSTOPPED`, or `WNOWAIT`, so omit those
from that platform.

Author: worr

src/sys/wait.rs

@@ -8,10 +8,13 @@ libc_bitflags!(
     pub struct WaitPidFlag: c_int {
         WNOHANG;
         WUNTRACED;
+        #[cfg(not(target_os = "openbsd"))]
         WEXITED;
         WCONTINUED;
+        #[cfg(not(target_os = "openbsd"))]
         WSTOPPED;
         /// Don't reap, just poll status.
+        #[cfg(not(target_os = "openbsd"))]
         WNOWAIT;
         /// Don't wait on children of other threads in this group
         #[cfg(any(target_os = "android", target_os = "linux"))]