builddeb: put the dbg files into the correct directory

Since the conversion of objtree to use relative pathnames (commit
7e1c0477, "kbuild: Use relative path for $(objtree)"), the debug
info files have been ending up in /debian/dbgtmp/ in the regular
linux-image package instead of the debug files package. Fix up the
paths so that the debug files end up in the -dbg package.

This is based on a similar patch by Darrick.

Reported-and-tested-by: "Darrick J. Wong" <darrick.wong@oracle.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>