Fix: ensure all probe providers have their symbols

Fix: ensure all probe providers have their symbols

Original commit log from stable-2.2:
Following the removal of metadata generation from UST, the probes cannot
find the tracepoint_dlopen() symbol. Ensure those probes can be built by
declaring this symbol within each probe.

stable-2.1 backport:
Similar issue was experienced with the "demo" test program when built
outside of the UST tree.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>