etc: add missing mtree entry for certctl tests

Otherwise we'll fail to create the appropriate hierarchy and blow up
when we go to actually install the test.

Reported by:	np
Fixes:		81d8827ad8 ("certctl: Reimplement in C")

etc/mtree/BSD.tests.dist

@@ -1255,6 +1255,8 @@
         ..
     ..
     usr.sbin
+        certctl
+        ..
         chown
         ..
         ctladm