um: fix call tracer and bug handler

Commit 1de1502c ("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 <richard@nod.at>
Reported-by: Sergei Trofimovich <slyich@gmail.com>
Tested-by: Sergei Trofimovich <slyich@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>