lib: don't depend on linux headers being installed.

gen_crc64table requires linux include files to be installed in
/usr/include/linux.  This is a new requrement so hosts that could
previously build the kernel, now cannot.

gen_crc64table makes this requirement by including <linux/swab.h>, but
nothing from that header is actaully used.

So remove the #include, so that the linux headers no longer need to be
installed.

Fixes: feba04fd ("lib: add crc64 calculation routines")
Signed-off-by: NeilBrown <neil@brown.name>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>