backports: make compat module use subsys_initcall()

backports: make compat module use subsys_initcall()

This is a no-op for the modular use case of the
backports project but by enabling this it allows
for developers a way to embed the backports releases
onto a target source Linux directory, with obvious
pending modifications, by adding our backport_init()
onto the target kernel's init/main.c start_kernel().

Enable this theoretical use case. Hackers welcome to
go play on this theoretical integration strategy now.

Cc: pstew@chromium.org
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>