builddeb: fix missing headers in linux-headers package

The kernel headers package (linux-headers) doesn't include
header files from other architectures required to build
out-of-tree modules.

For e.g. on ARM64, opcodes.h includes the same file from ARM
which causes the following error:
./arch/arm64/include/asm/opcodes.h:1:43: fatal error:
../../arm/include/asm/opcodes.h: No such file or directory
compilation terminated.

Signed-off-by: Azriel Samson <asamson@codeaurora.org>
Signed-off-by: Michal Marek <mmarek@suse.com>