main/lttng-ust: add depends_dev on userspace-rcu-dev
/usr/include/lttng/tracepoint-rcu.h:10:10: fatal error: urcu/compiler.h: No such file or directory
10 | #include <urcu/compiler.h>
| ^~~~~~~~~~~~~~~~~
/usr/include/lttng/tracepoint-rcu.h:10:10: fatal error: urcu/compiler.h: No such file or directory
10 | #include <urcu/compiler.h>
| ^~~~~~~~~~~~~~~~~