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.