RHELPLAN-171066 - add xxhash to gcc-misc

[fche]

cc: @hhorak

[siddhesh]

I reckon the xxhash entry also needs to be dropped from configs/sst_kernel_security-unwanted.yaml.

[hhorak]

cc: @hhorak

LGTM

[yselkowitz]

Based on the discussion above, this package is going to be buildroot-only for now. Therefore, I see no need to list it in CR, it will be (and in ELN already is) pulled in automatically as a build dependency of the affected packages (starting with dwz). If at some point in the future you decide to ship xxhash -- e.g. if a package starts using it NOT in header-only inline mode, causing xxhash-libs to be a runtime dependency, and in which case the other subpackages should go into CRB -- then this would be the correct change to make.