compat/mingw.h: define PRId64

Provide PRId64 alongside PRIuMAX.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/config-int-range-check'

* jk/config-int-range-check:
  compat/mingw.h: define PRId64