Make CopyNativeDependencies.targets Windows only

[nulltoken]

This blindly copies libgit2 binaries.

Even on Mono.... Yeah. I know... Shame.

cf #733 and #800

[Therzok]

Okay so, if we want this to happen, we would need to make two different configurations in LibGit2Sharp.

DebugWindows (Default)
DebugUnix (Mono)

And include the target running conditionally based on which configuration is selected.

[nulltoken]

Related #867

@Therzok How about:

• Rename the target as CopyWindowsNativeDependencies.targets
• Make the import depend on Condition=" ('$(OS)' == 'Windows_NT') "

[Therzok]

That works too, forgot about it being set by default. Or we can check $(OS) != 'Unix' too.