BlobGrid: Define virtual destructor in .cpp file

This fixes compiler warnings from clang:

src/textord/blobgrid.h:33:7: warning:
 'BlobGrid' 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/blobgrid.cpp

@@ -25,6 +25,11 @@ BlobGrid::BlobGrid(int gridsize, const ICOORD& bleft, const ICOORD& tright)
   : BBGrid<BLOBNBOX, BLOBNBOX_CLIST, BLOBNBOX_C_IT>(gridsize, bleft, tright) {
 }
 
+// Destructor.
+// It is defined here, so the compiler can create a single vtable
+// instead of weak vtables in every compilation unit.
+BlobGrid::~BlobGrid() = default;
+
 // Inserts all the blobs from the given list, with x and y spreading,
 // without removing from the source list, so ownership remains with the
 // source list.
@@ -37,5 +42,4 @@ void BlobGrid::InsertBlobList(BLOBNBOX_LIST* blobs) {
   }
 }
 
-
 }  // namespace tesseract.

src/textord/blobgrid.h

@@ -33,7 +33,7 @@ using BlobGridSearch = GridSearch<BLOBNBOX, BLOBNBOX_CLIST, BLOBNBOX_C_IT>;
 class BlobGrid : public BBGrid<BLOBNBOX, BLOBNBOX_CLIST, BLOBNBOX_C_IT> {
  public:
   BlobGrid(int gridsize, const ICOORD& bleft, const ICOORD& tright);
-  virtual ~BlobGrid() = default;
+  virtual ~BlobGrid();
 
   // Inserts all the blobs from the given list, with x and y spreading,
   // without removing from the source list, so ownership remains with the