Fix compiler warning (issue #25)

src/Fl_Table_Row.cxx:56:25: warning: argument 2 range
    [18446744071562067969, 18446744073709551615] exceeds maximum
    object size 9223372036854775807 [-Walloc-size-larger-than=]
     arr = (char*)realloc(arr, count * sizeof(char));
                  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes #25

src/Fl_Table_Row.cxx

@@ -53,7 +53,7 @@ Fl_Table_Row::CharVector::~CharVector() {	// DTOR
 
 void Fl_Table_Row::CharVector::size(int count) {
   if (count != _size) {
-    arr = (char*)realloc(arr, count * sizeof(char));
+    arr = (char*)realloc(arr, (unsigned)count * sizeof(char));
     _size = count;
   }
 }