um: fix call tracer and bug handler

Commit 1de1502 ("x86, um: now we can get rid of trivial uml headers")
removed accidentally bug.h which broke UML's call tracer and bug
handler.

Without asm-generic/bug.h UML uses BUG() from arch/x86/ which makes use
of ud2.  UML cannot use ud2, it raises SIGILL in user mode.  As UML has
a different stack for handling signals the call trace will be cut off.

Signed-off-by: Richard Weinberger <[email protected]>
Reported-by: Sergei Trofimovich <[email protected]>
Tested-by: Sergei Trofimovich <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: richardweinberger

arch/um/include/asm/bug.h

@@ -0,0 +1,6 @@
+#ifndef __UM_BUG_H
+#define __UM_BUG_H
+
+#include <asm-generic/bug.h>
+
+#endif