mingw workaround for QPDFExc destructor

mingw doesn't like it when you don't inline empty virtual destructors.

include/qpdf/QPDFExc.hh

@@ -39,7 +39,9 @@ class QPDFExc: public std::runtime_error
 	    qpdf_offset_t offset,
 	    std::string const& message);
     QPDF_DLL
-    virtual ~QPDFExc() throw ();
+    virtual ~QPDFExc() throw ()
+    {
+    }
 
     // To get a complete error string, call what(), provided by
     // std::exception.  The accessors below return the original values

libqpdf/QPDFExc.cc

@@ -15,10 +15,6 @@ QPDFExc::QPDFExc(qpdf_error_code_e error_code,
 {
 }
 
-QPDFExc::~QPDFExc() throw ()
-{
-}
-
 std::string
 QPDFExc::createWhat(std::string const& filename,
 		    std::string const& object,