fix: export absl::time_zone name on MinGW DLL builds

[mvanhorn]

Summary

Building the monolithic shared library (-DABSL_BUILD_MONOLITHIC_SHARED_LIBS=ON -DBUILD_SHARED_LIBS=ON) with MinGW on Windows fails at link time with an undefined reference to absl::time_internal::cctz::GetWindowsLocalTimeZone(). That symbol lives in time/internal/cctz/src/time_zone_name_win.cc, which CMake/AbseilDll.cmake only appends to ABSL_INTERNAL_DLL_FILES under if(MSVC) - so MinGW DLL builds never compile it, even though time_zone_lookup.cc calls it on all Windows toolchains.

This splits the single if(MSVC) ... else() ... endif() block into two independent conditions: the time_zone_name_win.cc/.h sources are appended under if(WIN32) (covering MSVC, clang-cl, and MinGW), and the flags/* source group stays under if(NOT MSVC) so MinGW keeps receiving exactly the flags sources it gets today.

Why this matters

The per-target (non-DLL) build in absl/time/CMakeLists.txt already gates this file on $<$<PLATFORM_ID:Windows>:...> - i.e. any Windows platform - so the DLL list's MSVC-only gate is the inconsistency. WIN32 is the correct CMake variable here; the patch sketched in the issue comments used if(Windows), which is not a variable CMake defines, so this supersedes it. A carry-along fix was merged downstream into microsoft/vcpkg (#50887), but upstream remains broken.

Testing

No C++ sources change and there is no test file for the DLL file list, so verification is a CMake configure/build matrix: the monolithic shared-library build now links under MinGW (the previously-undefined GetWindowsLocalTimeZone() is compiled into the DLL), and the MSVC and non-MSVC builds are unchanged (MSVC still gets the Windows time-zone source via WIN32; MinGW still gets the flags/* sources via NOT MSVC).

Fixes #2025