vircgroup: Fix build issue mingw cross compile

Commit id '2dbfa716' exposed virCgroupDetectMountsFromFile, but did not
add the corresponding entry in the "#else /* !VIR_CGROUP_SUPPORTED */"
section of the module.

src/util/vircgroup.c

@@ -4020,6 +4020,17 @@ virCgroupAvailable(void)
 }
 
 
+int
+virCgroupDetectMountsFromFile(virCgroupPtr group ATTRIBUTE_UNUSED,
+                              const char *path ATTRIBUTE_UNUSED,
+                              bool checkLinks ATTRIBUTE_UNUSED)
+{
+    virReportSystemError(ENXIO, "%s",
+                         _("Control groups not supported on this platform"));
+    return -1;
+}
+
+
 int
 virCgroupNewPartition(const char *path ATTRIBUTE_UNUSED,
                       bool create ATTRIBUTE_UNUSED,