cross-canadian: Whitelist "mingw32" as TARGET_OS

If a recipe inherits cross-canadian and contains "nativesdk" in
BBCLASSEXTEND and meta-mingw is included and multiconfig is enabled,
bitbake will generate the correspending recipe. As meta-mingw sets
SDK_OS to "mingw32", that's what TARGET_OS will be set to as well.
Thus, currently such a recipe won't pass the check and fail with
a message:

Building cross-candian for an unknown TARGET_SYS
(x86_64-mysdk-mingw32), please update cross-canadian.bbclass

Even when building an SDK targeting Linux, but the mentioned conditions
are met, bitbake will try to generate the corresponding recipe and fail.

As the described combination seems valid, including "mingw32" into the
whitelist unconditionally as a fix is suggested.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>