Eliminate missing virtual destructor warning on FreeBSD

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

llvm-svn: 184742
diff --git a/lldb/source/Plugins/Process/FreeBSD/ProcessMonitor.cpp b/lldb/source/Plugins/Process/FreeBSD/ProcessMonitor.cpp
index cc412ed..e5bd4fb 100644
--- a/lldb/source/Plugins/Process/FreeBSD/ProcessMonitor.cpp
+++ b/lldb/source/Plugins/Process/FreeBSD/ProcessMonitor.cpp
@@ -204,6 +204,7 @@
 class Operation
 {
 public:
+    virtual ~Operation() {}
     virtual void Execute(ProcessMonitor *monitor) = 0;
 };