Avoid mixing designated and non-designated field initializers.

Avoid mixing designated and non-designated field initializers.

As revised by commit 9324c8c58, PG_MODULE_MAGIC constructed a
struct initializer containing both designated fields and a
non-designated "0".  That's okay in C, but not in C++, with
the result that extensions written in C++ failed to compile.
Change it to use only designated field initializers.

Author: Yurii Rashkovskii 
Discussion: https://postgr.es/m/CAG=VW14mctsR543gpzLCuJ9JgJqwa=ptmBfGvxEjs+k8Jf7-Bg@mail.gmail.com