Unbreak MPI compilation.

3c2c7b01bd deleted the TTCache default constructor, but we need it
when MPI is active. This wasn't discovered during the merging,
and it is hard to put it properly into the commit history due to
rebase/rerere shortcomings. (This means that some of the merge
commits will not compile with MPI on, and this patch needs to be
added on top if bisecting.)

src/tt.h

@@ -56,7 +56,12 @@ struct TTData {
     Bound bound;
     bool  is_pv;
 
+#ifdef USE_MPI
+    // We need this for TTCache to be constructible.
+    TTData() = default;
+#else
     TTData() = delete;
+#endif
 
     // clang-format off
     TTData(Move m, Value v, Value ev, Depth d, Bound b, bool pv) :