bin: compile lttng-consumerd as a C++

Same as the previous commits, but change lttng-consumerd to be a C++
program.

Function lttng_consumer_get_type is dlsym-ed by
tests/regression/tools/notification/consumer_testpoints.c, so we have
two choices: make the dlsym use the mangled name or make the function
have a C linkage name. The latter is better, because the mangled name
is implementation-defined, and therefore more fragile.

Change-Id: I1986e38c193a935721e52c89426c89fd434ccd6b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>