MODSIGN: Fix build error with strict typechecking

MODSIGN: Fix build error with strict typechecking

"MODSIGN: Provide module signing public keys to the kernel" causes a build
error when CONFIG_UIDGID_STRICT_TYPE_CHECKS is enabled:

kernel/modsign_pubkey.c: In function 'module_verify_init':
kernel/modsign_pubkey.c:47: error: incompatible type for argument 3 of 'key_alloc'
include/linux/key.h:195: note: expected 'kuid_t' but argument is of type 'int'
kernel/modsign_pubkey.c:47: error: incompatible type for argument 4 of 'key_alloc'
include/linux/key.h:195: note: expected 'kgid_t' but argument is of type 'int'

Cast the parameters accordingly.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>