launch: make px4 required (#9682)

launch/posix_sitl.launch

@@ -26,7 +26,7 @@
     <!-- PX4 SITL -->
     <arg unless="$(arg interactive)" name="px4_command_arg1" value="-d"/>
     <arg     if="$(arg interactive)" name="px4_command_arg1" value=""/>
-    <node name="sitl" pkg="px4" type="px4" output="screen" args="$(find px4) $(arg rcS) $(arg px4_command_arg1)"/>
+    <node name="sitl" pkg="px4" type="px4" output="screen" args="$(find px4) $(arg rcS) $(arg px4_command_arg1)" required="true"/>
     <!-- Gazebo sim -->
     <include file="$(find gazebo_ros)/launch/empty_world.launch">
         <arg name="gui" value="$(arg gui)"/>