Makefile.uk: Fix gcc specific warnings

Makefile.uk: Fix gcc specific warnings

-Wno-restrict, Wno-unused-but-set-variable, -Wno-maybe-unitialized and
-Wno-format-contains-nul are gcc-specific flags, so include them only when
compiling with gcc and not with clang.

Signed-off-by: Miguel Rodrigues <migb.rodrigues@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #51