Fix -Wcast-function-type warnings on Windows/MinGW

Fix -Wcast-function-type warnings on Windows/MinGW

After de8feb1f3a23465b5737e8a8c160e8ca62f61339, some warnings remained
that were only visible when using GCC on Windows.  Fix those as well.

Note that the ecpg test source files don't use the full pg_config.h,
so we can't use pg_funcptr_t there but have to do it the long way.