mingw: use strftime() directly in UCRT builds

mingw: use strftime() directly in UCRT builds

The `mingw_strftime()` wrapper exists to work around msvcrt.dll's
incomplete `strftime()` implementation by dynamically loading the
version from ucrtbase.dll at runtime via `LoadLibrary()` +
`GetProcAddress()`. When the binary is already linked against UCRT
(i.e. when building in the UCRT64 environment), the linked-in
`strftime()` is the ucrtbase.dll version, making the dynamic loading
needless churn: It's calling the very same code.

Simply guard both the declaration and implementation so that the
unnecessary work-around is skipped in UCRT builds.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>