Adds package requirements to Common mistakes

I made this mistake, and would like to update the documentation
accordingly.

Signed-off-by: John Edward Broadbent <jebr@google.com>
Change-Id: Ic0c477c62881a04ef731a5dc3827cfabad84cd32
diff --git a/security/TLS-configuration.md b/security/TLS-configuration.md
index 1eacc25..3d1067a 100644
--- a/security/TLS-configuration.md
+++ b/security/TLS-configuration.md
@@ -425,3 +425,5 @@
 [Verify certificates](#Verify-certificates).
 
 * Attempting to load the same certificate twice will end up with an error.
+
+* Not having phosphor-bmcweb-cert-config in the build.