scripts: gen_syscalls: declare syscalls with void instead of zero args

In C, `void foo(void);` and `void foo();` mean subtly different things.
The former means "foo takes zero arguments"; the latter means
"foo takes an unspecified number of arguments". This can result in
calling convention mismatches in exceptional cases.

Change to emitting `(void)` instead of `()` for a syscall with
zero arguments.

Signed-off-by: James Harris <james.harris@intel.com>