Config.uk: Allow deselection of ukmmap

Config.uk: Allow deselection of ukmmap

With posix-mmap we have an alternative implementation of the memory
management functions such as mmap, mprotect, etc. To be able to select
posix-mmap, we must deselect ukmmap. Thus, this commit removes the
hard dependency.

Signed-off-by: Marc Rittinghaus <marc.rittinghaus@unikraft.io>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #52