CUtil: Define virtual destructor in .cpp file

This fixes compiler warnings from clang:

src/cutil/cutil_class.h:27:7: warning:
 'CUtil' 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/cutil/cutil_class.cpp

@@ -20,4 +20,9 @@
 
 namespace tesseract {
 
+// Destructor.
+// It is defined here, so the compiler can create a single vtable
+// instead of weak vtables in every compilation unit.
+CUtil::~CUtil() = default;
+
 }  // namespace tesseract

src/cutil/cutil_class.h

@@ -27,7 +27,7 @@ namespace tesseract {
 class CUtil : public CCUtil {
  public:
   CUtil() = default;
-  ~CUtil() = default;
+  virtual ~CUtil();
   void read_variables(const char *filename, bool global_only);
 };