[xbuild] Fix dependency in C# targets

There was a typo in DependsOnTargets attribute of the
_AddCorlibReference target definition. It didn't break any
behaviour, since the target it depended on was always built
earlier in the build anyway, so it was really a sanity check.

However, it broke the tests for the new (unused) engine,
Microsoft.Build.dll, which checks for unexpected attributes.