Eliminate missing virtual destructor warning on FreeBSD

Akin to r181712 (88e529b7) of Linux/ProcessMonitor.cpp

llvm-svn: 184742

lldb/source/Plugins/Process/FreeBSD/ProcessMonitor.cpp

@@ -204,6 +204,7 @@ EnsureFDFlags(int fd, int flags, Error &error)
 class Operation
 {
 public:
+    virtual ~Operation() {}
     virtual void Execute(ProcessMonitor *monitor) = 0;
 };