libnfnetlink: use a correct LIC_FILES_CHKSUM

    The current LIC_FILES_CHKSUM checks the copyright/license from the
    generated Makefile.in, this is not correct.
    Since there is no seperated license/copying file, this fixes it to
    use license content in src/libnfnetlink.c instead.

Signed-off-by: Zhenhua Luo <b19537@freescale.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>