Fix intrin0.h include guard.

This lighter-weight intrinsics header is available starting
with Microsoft Visual Studio 2017, so the previous change
to allow this header failed when building with Visual
Studio 2015.

Restores the appveyor continuous integration build.
4 jobs for intrin0 in 4 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10299
docker
autoconf

00:04:47

passed #10300
docker
cmake

00:02:14

 
  External
passed continuous-integration/appveyor/branch

passed continuous-integration/appveyor/merge-request