CI: Add 64bit build to AppVeyor yml.

Signed-off-by: Philip Tricca <philip.b.tricca@intel.com>
diff --git a/.appveyor.yml b/.appveyor.yml
index 939d118..369af3c 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -7,6 +7,7 @@
   - Debug
 platform:
   - x86
+  - x64
 build:
   project: tpm2-tss.sln
   parallel: true