UBUNTU: [debian] derive indep_hdrs_pkg_name from src_pkg_name

UBUNTU: [debian] derive indep_hdrs_pkg_name from src_pkg_name

This long-standing oversight in our debian rules hardcodes the string "linux"
instead of using the $(src_pkg_name) for just one of the generated .deb package
names: linux-headers-x.x.x-x.  Lets fix it in the generic branches
(T,X,Y,Z,unstable) so that we won't have to keep applying this patch to each of
the derivative/custom kernels.

-----8<-----

Ignore: yes

Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>