[libcxx] [test] Fix the get_temp_file_name() function for mingw

Add the missing includes for getting the defines and functions used
in the mingw version of get_temp_file_name().

This fixes 31 tests when built in a mingw configuration.

Also remove a redundant ifdef; _WIN32 is defined in mingw targets too.

Differential Revision: https://reviews.llvm.org/D97456