builddeb: include objtool binary in headers package

"objtool" is required for building external m dules if "Compile-time
stack metadata validation" is enabled.  Otherwise all builds based
on the headers package fail with:

 make[1]: Entering directory '/usr/src/linux-headers-4.6.0-rc6'
 make[2]: *** No rule to make target 'tools/objtool/objtool', needed by 'foo.o'.  Stop.
 Makefile:1598: recipe for target 'foo.ko' failed
 make[1]: *** [foo.ko] Error 2
 make[1]: Leaving directory '/usr/src/linux-headers-4.6.0-rc6'

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Michal Marek <mmarek@suse.com>