Minor fix: make sure alter table wont try to change our storage engine to something else.

storage/oqgraph/ha_oqgraph.h

@@ -78,6 +78,9 @@ public:
   double read_time(uint index, uint ranges, ha_rows rows)
   { return 1; }
 
+  // Doesn't make sense to change the engine on a virtual table.
+  virtual bool can_switch_engines() { return false; }
+
   int open(const char *name, int mode, uint test_if_locked);
   int close(void);
   int write_row(byte * buf);