[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

libcxx/test/support/platform_support.h

@@ -56,8 +56,10 @@
 #include <codecvt>
 #include <locale>
 #include <string>
-#if defined(_WIN32) || defined(__MINGW32__)
+#if defined(_WIN32)
+#   define WIN32_LEAN_AND_MEAN // Reduce overhead of including windows.h
 #   include <io.h> // _mktemp_s
+#   include <windows.h> // MAX_PATH, GetTempPath, GetTempFileName
 #else
 #   include <unistd.h> // close
 #endif