kbuild: generate generic headers before recursing into scripts

The headers are now needed inside scripts/mod since 6543becf
("mod/file2alias: make modalias generation safe for cross compiling").

Reported-and-tested-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Reported-and-tested-by: "Jon Medhurst (Tixy)" <tixy@linaro.org>
Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>