arch: Add a virtual destructor to BaseHTMCheckpoint.

Since it has virtual methods, it should also have a virtual destructor.
My compiler complains otherwise, which breaks my build.

Change-Id: I44bba97b76931bab6e3511fcdee79831080c12d4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34215
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/generic/htm.hh

@@ -137,6 +137,7 @@ class BaseHTMCheckpoint
     {
         reset();
     }
+    virtual ~BaseHTMCheckpoint() {}
 
     /**
      * Every ISA implementing HTM support should override the