AlignedBlob: Define virtual destructor in .cpp file

This fixes compiler warnings from clang:

src/textord/alignedblob.h:81:7: warning:
 'AlignedBlob' 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/textord/alignedblob.cpp

@@ -212,6 +212,11 @@ static bool AtLeast2LineCrossings(BLOBNBOX_CLIST* blobs) {
   return total_crossings >= 2;
 }
 
+// Destructor.
+// It is defined here, so the compiler can create a single vtable
+// instead of weak vtables in every compilation unit.
+AlignedBlob::~AlignedBlob() = default;
+
 // Finds a vector corresponding to a set of vertically aligned blob edges
 // running through the given box. The type of vector returned and the
 // search parameters are determined by the AlignedBlobParams.

src/textord/alignedblob.h

@@ -81,7 +81,7 @@ struct AlignedBlobParams {
 class AlignedBlob : public BlobGrid {
  public:
   AlignedBlob(int gridsize, const ICOORD& bleft, const ICOORD& tright);
-  virtual ~AlignedBlob() = default;
+  virtual ~AlignedBlob();
 
   // Return true if the given coordinates are within the test rectangle
   // and the debug level is at least the given detail level.