build: Turn off parallel builds in Appveyor

Because tss2-tcti-mssim and tss2-sys are both dependent on tss2-mu, they
can not be built in parallel or else there is a chance that they will
run into issues when trying to build tss2-mu at the same time.

Signed-off-by: David Maria <davidjmaria@fb.com>
diff --git a/.appveyor.yml b/.appveyor.yml
index 49bf7de..5ea3f7c 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -11,5 +11,5 @@
   - x64
 build:
   project: tpm2-tss.sln
-  parallel: true
+  parallel: false
   verbosity: normal