tree-wide: voidify reboot() invocations

We use (void) in most cases for reboot() already, let's add it to the
others as well.

src/core/emergency-action.c

@@ -93,7 +93,7 @@ int emergency_action(
                 }
 
                 log_info("Rebooting.");
-                reboot(RB_AUTOBOOT);
+                (void) reboot(RB_AUTOBOOT);
                 break;
 
         case EMERGENCY_ACTION_POWEROFF:
@@ -112,7 +112,7 @@ int emergency_action(
                 sync();
 
                 log_info("Powering off.");
-                reboot(RB_POWER_OFF);
+                (void) reboot(RB_POWER_OFF);
                 break;
 
         default:

src/core/shutdown.c

@@ -548,7 +548,7 @@ int main(int argc, char *argv[]) {
                 assert_not_reached("Unknown magic");
         }
 
-        reboot(cmd);
+        (void) reboot(cmd);
         if (errno == EPERM && in_container) {
                 /* If we are in a container, and we lacked
                  * CAP_SYS_BOOT just exit, this will kill our