lttng-tools returns incomplete type error

Question

I'm trying to build lttng-tools and I keep getting this

../../../src/common/sessiond-comm/sessiond-comm.h:713:28: error: field ‘channel’ has incomplete type
  713 |   struct lttng_ust_channel channel;
      |                            ^~~~~~~
../../../src/common/sessiond-comm/sessiond-comm.h:714:27: error: field ‘stream’ has incomplete type
  714 |   struct lttng_ust_stream stream;
      |                           ^~~~~~
../../../src/common/sessiond-comm/sessiond-comm.h:715:26: error: field ‘event’ has incomplete type
  715 |   struct lttng_ust_event event;
      |                          ^~~~~
../../../src/common/sessiond-comm/sessiond-comm.h:716:28: error: field ‘context’ has incomplete type
  716 |   struct lttng_ust_context context;
      |                            ^~~~~~~
../../../src/common/sessiond-comm/sessiond-comm.h:717:35: error: field ‘version’ has incomplete type
  717 |   struct lttng_ust_tracer_version version;

I haven't found any forward declarations. Is it possible to make GCC show me where it is?