util: Fix build without devmapper

stdlib.h header file needed for getenv was only transitively included
through libdevmapper.h.

src/util/virutil.c

@@ -26,6 +26,7 @@
 
 #include <config.h>
 
+#include <stdlib.h>
 #include <dirent.h>
 #include <stdio.h>
 #include <stdarg.h>