package/linux-headers: add comment to safely ignore missing cross-compiler

For some architectures (eg. Arc, Cris, Hexagon, ia64, Parisc, Score and
Xtensa), the Linux buildsystem tries to call the cross-compiler when
installing the headers.

This is a spurious call, since a cross-compiler is not needed at all to
install the headers.

As some users have reported the issue, just add a comment in linux-headers.mk
directing the user to ignore those errors.

Reported-by: Noam Camus <noamc@ezchip.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>