MATRIX: Define virtual destructor in .cpp file

This fixes compiler warnings from clang:

src/ccstruct/matrix.h:575:7: warning:
 'MATRIX' has no out-of-line virtual method definitions;
 its vtable will be emitted in every translation unit [-Wweak-vtables]

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/ccstruct/matrix.cpp

@@ -32,6 +32,11 @@
 #include "tprintf.h"
 #include "unicharset.h"
 
+// Destructor.
+// It is defined here, so the compiler can create a single vtable
+// instead of weak vtables in every compilation unit.
+MATRIX::~MATRIX() = default;
+
 // Returns true if there are any real classification results.
 bool MATRIX::Classified(int col, int row, int wildcard_id) const {
   if (get(col, row) == NOT_CLASSIFIED) return false;

src/ccstruct/matrix.h

@@ -577,6 +577,8 @@ class MATRIX : public BandTriMatrix<BLOB_CHOICE_LIST *> {
   MATRIX(int dimension, int bandwidth)
     : BandTriMatrix<BLOB_CHOICE_LIST *>(dimension, bandwidth, NOT_CLASSIFIED) {}
 
+  virtual ~MATRIX();
+
   // Returns true if there are any real classification results.
   bool Classified(int col, int row, int wildcard_id) const;