sh: fix function signature of cpu_coregroup_mask to match pointer type

The signedness mismatch of the argument type produces an error
compiling kernel/sched/core.c with -Werror=incompatible-pointer-types,
which is now used by default.

Fixes: ea8daa7b "kbuild: Add option to turn incompatible pointer check into error"
Signed-off-by: Rich Felker <dalias@libc.org>