Makefile.uk: Use -Wno-cast-function-type for GCC >= 8

Makefile.uk: Use -Wno-cast-function-type for GCC >= 8

The build option `-Wno-cast-function-type`, used in `Makefile.uk` is
available from GCC >= 8. When used in GCC <= 7, it issues a warning.

This commit updates the build flags to enable `-Wno-cast-function-type`
only when using GCC >= 8.

GitHub-Fixes: #27

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Cezar Craciunoiu <cezar.craciunoiu@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #28