C++ compatability of headers

[csehydrogen]

Although UCX is C library, all headers of UCX can be included in C++ without any problems.
In xucg, however, one error prevents headers to be included in C++.

api/ucg_mpi.h:118:5: error: designator order for field ‘ucg_collective::<unnamed struct>::<unnamed union>::op’ does not match declaration order in ‘ucg_collective::<unnamed struct>’

This is due to the fact that C++ does not support out-of-order designated initializer, while C allows.
This can be solved by initializing ucg_collective_params_t in-order.