Fix: make hello.cxx compile with g++

Move enumeration definition out of lttng_ust_lib_ring_buffer_config to
make them visible at global scope for C++ compilers.

Modify designated initializers: reordering initializers, add missing
initializers, reformat nested initializers, in order to make g++ compile.

Fixes #338

Signed-off-by: Zifei Tong <soariez@gmail.com>