package/linux-headers: fix support for -rc kernels

package/linux-headers: fix support for -rc kernels

-rc kernels after v3.x are no longer available in the testing
subdirectory. Instead they should be fetched from cgit.

Commit ff4cccbdcf5417a5a8c251ff302791a306023cc1 did this for linux
itself, now we also do it for linux-headers.

When fetched from cgit, .tar.xz can't be used. Adding this to the
existing condition is not so simple, so refactor how _SOURCE is set:
simply set it explicitly in each branch of the condition. While more
verbose (it is repeated 4 times), it's easier to understand and to
maintain.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>