skip ucontext.h

Index: rts/System/Platform/Linux/ThreadSupport.h
--- rts/System/Platform/Linux/ThreadSupport.h.orig
+++ rts/System/Platform/Linux/ThreadSupport.h
@@ -7,7 +7,9 @@
 	// FIXME: exclusively for ucontext.h
 	#define _XOPEN_SOURCE 700
 #endif
-#include <ucontext.h>
+#if !defined(__OpenBSD__)
+    #include <ucontext.h>
+#endif
 #include <functional>
 #include <memory>
 
