libzpool: force inlining of xxhash after removing xxhash.c

libzpool had a number of undefined symbols related to xxhash after
xxhash.c was removed from the build.

Fixes: 8a62a2a5659d ("zfs: merge openzfs/zfs@f8e5af53e")
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D56770