maint: add virCgroupFree to the list of free-like functions

This makes the useless-if-before-free test in maint.mk spot
uses of virCgroupFree just like it does for free and the other
listed functions.
* cfg.mk (useless_free_options): Add virCgroupFree.
Prompted by suggestion from Eric Blake.

cfg.mk

@@ -66,6 +66,7 @@ local-checks-to-skip =			\
 
 useless_free_options =		\
   --name=sexpr_free		\
+  --name=virCgroupFree		\
   --name=VIR_FREE		\
   --name=xmlFree		\
   --name=xmlXPathFreeContext	\