Use statvfs() on OpenBSD.

Index: source/blender/blenlib/intern/storage.c
--- source/blender/blenlib/intern/storage.c.orig
+++ source/blender/blenlib/intern/storage.c
@@ -29,7 +29,7 @@
 
 #include <sys/stat.h>
 
-#if defined(__NetBSD__) || defined(__DragonFly__) || defined(__HAIKU__)
+#if defined(__NetBSD__) || defined(__DragonFly__) || defined(__HAIKU__) || defined(__OpenBSD__)
 /* Other modern unix OS's should probably use this also. */
 #  include <sys/statvfs.h>
 #  define USE_STATFS_STATVFS
