[3.12] gh-127906: Declare timeval struct in pytime.h on Windows

[vstinner]

Fix the following MSVC compiler warning:

include\cpython\pytime.h(192): warning C4115: 'timeval':
named type definition in parentheses

• Issue: [tests] Backport test_cext and test_cppext (C API tests) from main to the 3.12 branch #127906

[vstinner]

Note: the existing struct timeval code was added for clang by commit 737d367:

+#ifdef __clang__
+struct timeval;
+#endif

commit 737d367b1f4bad76914173a64d6bbe19a984cd5f
Author: Steve Dower <[email protected]>
Date:   Fri Jan 27 14:45:08 2023 +0000

    gh-77532: Minor tweaks to allow compiling with PlatformToolset=ClangCL on Windows (GH-101352)
    
    To use this, ensure that clang support was selected in Visual Studio Installer, then set the PlatformToolset environment variable to "ClangCL" and build as normal from the command line.
    It remains unsupported, but at least is possible now for experimentation.