core: fix the return type for xxx_running_timeout() functions

otherwise we might return an invalid value, since `usec_t` is 64-bit,
whereas `int` might not be.

Follow-up to: 5918a93
Fixes: #20872

src/core/scope.c

@@ -52,7 +52,7 @@ static void scope_done(Unit *u) {
         s->timer_event_source = sd_event_source_disable_unref(s->timer_event_source);
 }
 
-static int scope_running_timeout(Scope *s) {
+static usec_t scope_running_timeout(Scope *s) {
         usec_t delta = 0;
 
         assert(s);

src/core/service.c

@@ -515,7 +515,7 @@ static void service_remove_fd_store(Service *s, const char *name) {
         }
 }
 
-static int service_running_timeout(Service *s) {
+static usec_t service_running_timeout(Service *s) {
         usec_t delta = 0;
 
         assert(s);