meson: fix compat sources when compiling with MSVC

meson: fix compat sources when compiling with MSVC

In our compat library we have both "msvc.c" and "mingw.c". The former is
mostly a thin wrapper around the latter as it directly includes it, but
it has a couple of extra headers that aren't included in "mingw.c" and
is expected to be used with the Visual Studio compiler toolchain.

While our Makefile knows to pick up the correct file depending on
whether or not the Visual Studio toolchain is used, we don't do the same
with Meson. Fix this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>