extension_datetime: Fix reference to MSVC_BUILTIN
extension_datetime: Fix reference to MSVC_BUILTIN

This commit fixes typo introduced in e67299c (Exclude timemodule.c when
building datemodule extension on windows.)

thewtex/python-cmake-buildsystem,python-cmake-buildsystem/python-cmake-buildsystem,thewtex/python-cmake-buildsystem,davidsansome/python-cmake-buildsystem,adrianbroher/python-cmake-buildsystem