objtool: Upgrade libelf-devel warning to error for CONFIG_ORC_UNWINDER

With CONFIG_ORC_UNWINDER, if the user doesn't have libelf-devel
installed, and they don't see the make warning, their ORC unwinder will
be silently broken.  Upgrade the warning to an error.

Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.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/d9dfc39fb8240998820f9efb233d283a1ee96084.1507079417.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>