LinuxKPI: ipv6.h add missing #include

Include linux/bitops.h for a definition of BITS_PER_LONG so that this
file can be used independently.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30429

(cherry picked from commit 762efb2d6dd67f7b999d40b10d58828efb473236)