Specialize read_float_or_quotient for mpq_class.

Some solution based on Fraction_traits would be more elegant, but this
is how it is done for leda_rational, so let us remain consistent for
now.

Number_types/include/CGAL/mpq_class.h

@@ -273,6 +273,19 @@ public:
     }
 };
 
+// Copied from leda_rational.h
+namespace internal {
+  // See: Stream_support/include/CGAL/IO/io.h
+  template <typename ET>
+  void read_float_or_quotient(std::istream & is, ET& et);
+
+  template <>
+  inline void read_float_or_quotient(std::istream & is, mpq_class& et)
+  {
+    internal::read_float_or_quotient<mpz_class,mpq_class>(is, et);
+  }
+} // namespace internal
+
 } //namespace CGAL
 
 #undef CGAL_CHECK_GMP_EXPR