kbuild: prevent make from deleting _shipped files

commit 7373f4f (kbuild: add implicit rules for parser generation)
created a implicit rule chain (%.c: %.c_shipped: %.y).
Make considers the _shipped files to be intermediate files which
causes them to be deleted if they didn't exist before make was run.
Mark the _shipped files PRECIOUS to prevent make from deleting them.

Signed-off-by: Peter Foley <pefoley2@verizon.net>
Acked-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>