patch 8.2.4532: suspending with CTRL-Z does not work on OpenBSD

Problem:    Suspending with CTRL-Z does not work on OpenBSD.
Solution:   Adjust #ifdef for SIGTSTP. (Stuart Henderson, closes #9912)
diff --git a/src/os_unix.c b/src/os_unix.c
index 58f8276..e617b12 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -887,8 +887,9 @@
     else
 	got_tstp = TRUE;
 
-#ifndef __ANDROID__
-    // this is not required on all systems
+#if !defined(__ANDROID__) && !defined(__OpenBSD__)
+    // This is not required on all systems.  On some systems (at least Android
+    // and OpenBSD) this breaks suspending with CTRL-Z.
     signal(SIGTSTP, (RETSIGTYPE (*)())sig_tstp);
 #endif
     SIGRETURN;