tools: Restore export.h

Commit 3f735377 ("tools: Copy lib/rbtree.c to tools/lib/") has
removed export.h, which was still in use by liblockdep. Restore
it.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1440479985-6696-1-git-send-email-sasha.levin@oracle.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>