fix build on OpenBSD/arm

[Stefan Sperling <st...@apache.org>]

APR is failing to build on OpenBSD/arm.

See https://marc.info/?l=openbsd-ports&m=152451633123867&w=2

This patch fixes it. However, I would hesitate using such an approach
in the first place, because you will receive more patches for someone
else's FavouriteBSD or some other OS in the future. I don't understand
what problem this check is really trying to solve, though, so I cannot
suggest a better solution unfortunately.

Index: include/apr_general.h
===================================================================
--- include/apr_general.h	(revision 1829959)
+++ include/apr_general.h	(working copy)
@@ -79,7 +79,7 @@ typedef enum { APR_WAIT_READ, APR_WAIT_WRITE } apr
  * @return offset
  */
 
-#if defined(CRAY) || (defined(__arm) && !(defined(LINUX) || defined(__FreeBSD__)))
+#if defined(CRAY) || (defined(__arm) && !(defined(LINUX) || defined(__FreeBSD__) || defined(__OpenBSD__)))
 #ifdef __STDC__
 #define APR_OFFSET(p_type,field) _Offsetof(p_type,field)
 #else