mingw: set _WIN32_WINNT explicitly for Git for Windows

mingw: set _WIN32_WINNT explicitly for Git for Windows

Previously, we only ever declared a target Windows version if compiling
with Visual C.

Which meant that we were relying on the MinGW headers to guess which
Windows version we want to target...

Let's be explicit about it, in particular because we actually want to
bump the target Windows version to Vista (which we will do in the next
commit).

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>