Add missing include guards into headers in lib/Headers. While it may appear
that these headers should not be included more than once, they are in fact
included twice when building our builtins module (in order for it to generate
submodules for them), and without this, any modular build enabling AVX and
including any builtin header fails.

Testing this is tricky because including any of these headers in a modular
build is liable to fail, due to unrelated builtin headers in the same module
including headers which might not be available on the system running the tests.
Suggestion on that front are welcome (but we're getting close to being able to
run a buildbot that has modules enabled for all tests, which would nicely solve
the testing problem).