Fixed license for all basic types

Just a simple fix and nobody will bother to do it next time.
OpenTK follows a similar approach, where [LICENSE.txt](
https://github.com/opentk/opentk/blob/develop/Documentation/License.txt)
contains a "Third Parties" section.

Unless there is a good reason to relicense, I'd suggest simply reproducing
the MIT license and leaving it at that. Considering the amount of
third-party dependencies in MonoGame, having a few source files under a
different license will not make any practical difference.