include/linux/idr.h: add #include <linux/bug.h>

include/linux/idr.h: add #include <linux/bug.h>

The <linux/bug.h> was removed from radix-tree.h by commit f5bba9d11a25
("include/linux/radix-tree.h: remove unneeded #include <linux/bug.h>").

Since that commit, tools/testing/radix-tree/ couldn't pass compilation
due to tools/testing/radix-tree/idr.c:17: undefined reference to
WARN_ON_ONCE.  This patch adds the bug.h header to idr.h to solve the
issue.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: f5bba9d11a2 ("include/linux/radix-tree.h: remove unneeded #include <linux/bug.h>")
Signed-off-by: Wei Wang <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>