Add missing `struct` keyword to the test p2-2.cpp

While working on D53847, I noticed that this test would fail once we
started recognizing the types in the modified `export` statement [0].
The tests would fail because Clang would emit a "declaration does not
declare anything" diagnostic instead of the expected namespace scope
diagnostic.

I believe that the test is currently incorrectly passing because Clang
doesn't parse the type and therefore doesn't treat the statement as a
declaration. My understanding is that the intention of this test case is
that it wants to export a `struct` type, which I believe requires a
`struct` keyword, even for types with template parameters. With this
change, the only error with these two statements should be the
namespace scope issue.

[0]: https://reviews.llvm.org/D53847?id=462032#inline-1297053

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D134578