remove inline from ConnectionBaseNodelet::isSubscribed()

jsk_peceptino could not find isSubscribed if it is defined as inline

```
usr/lib/x86_64-linux-gnu/libopencv_phase_unwrapping.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_optflow.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_ximgproc.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_video.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_videoio.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_imgcodecs.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_objdetect.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_calib3d.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_dnn.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_features2d.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_flann.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_photo.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_imgproc.so.4.5.4d /usr/lib/x86_64-linux-gnu/libopencv_core.so.4.5.4d
2025-01-05T05:23:12.3723125Z /usr/bin/ld: /tmp/ccUI43It.ltrans0.ltrans.o:(.data.rel.ro+0xc08): undefined reference to jsk_topic_tools::ConnectionBasedNodelet::isSubscribed()'
2025-01-05T05:23:12.3918628Z /usr/bin/ld: /tmp/ccUI43It.ltrans7.ltrans.o:(.data.rel.ro+0x578): undefined reference to jsk_topic_tools::ConnectionBasedNodelet::isSubscribed()'
2025-01-05T05:23:12.4099509Z /usr/bin/ld: /tmp/ccUI43It.ltrans14.ltrans.o:(.data.rel.ro+0x178): undefined reference to jsk_topic_tools::ConnectionBasedNodelet::isSubscribed()' follow
```

jsk_topic_tools/src/connection_based_nodelet.cpp

@@ -88,7 +88,7 @@ namespace jsk_topic_tools
     }
   }
 
-  inline bool ConnectionBasedNodelet::isSubscribed()
+  bool ConnectionBasedNodelet::isSubscribed()
   {
     return connection_status_ == SUBSCRIBED;
   }