tools/sched_ext: fix getopt() option variable signedness

Four example schedulers (scx_simple, scx_cpu0, scx_sdt, scx_userland)
declare the variable that holds getopt()'s return value as __u32.
getopt() returns int and uses -1 to mark end-of-options; storing that
sentinel in an unsigned variable turns it into 0xffffffff, and the
subsequent 'opt != -1' test only happens to keep working because both
operands of != are promoted to the same 0xffffffff.

Declare the variable as __s32 instead, matching getopt()'s actual
contract and the style already used in scx_qmap.c (int opt) and
scx_pair/central/flatcg.c (__s32 opt). This also silences the
-Wsign-compare warnings emitted for the affected files.

Signed-off-by: Liang Luo <luoliang@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/sched_ext/scx_cpu0.c

@@ -62,7 +62,7 @@ int main(int argc, char **argv)
 {
 	struct scx_cpu0 *skel;
 	struct bpf_link *link;
-	__u32 opt;
+	__s32 opt;
 	__u64 ecode;
 
 	libbpf_set_print(libbpf_print_fn);

tools/sched_ext/scx_sdt.c

@@ -44,7 +44,7 @@ int main(int argc, char **argv)
 {
 	struct scx_sdt *skel;
 	struct bpf_link *link;
-	__u32 opt;
+	__s32 opt;
 	__u64 ecode;
 
 	libbpf_set_print(libbpf_print_fn);

tools/sched_ext/scx_simple.c

@@ -64,7 +64,7 @@ int main(int argc, char **argv)
 {
 	struct scx_simple *skel;
 	struct bpf_link *link;
-	__u32 opt;
+	__s32 opt;
 	__u64 ecode;
 
 	libbpf_set_print(libbpf_print_fn);

tools/sched_ext/scx_userland.c

@@ -326,7 +326,7 @@ static int spawn_stats_thread(void)
 static void pre_bootstrap(int argc, char **argv)
 {
 	int err;
-	__u32 opt;
+	__s32 opt;
 	struct sched_param sched_param = {
 		.sched_priority = sched_get_priority_max(SCHED_EXT),
 	};