Add missing friend declaration

Fixes issue #100

Author: sehe

include/boost/dynamic_bitset/dynamic_bitset.hpp

@@ -1440,6 +1440,8 @@ class bit_iterator_base
     friend BOOST_DYNAMIC_BITSET_CONSTEXPR20 bool operator==( const bit_iterator_base< Iter > & lhs, const bit_iterator_base< Iter > & rhs );
     template< typename Iter >
     friend BOOST_DYNAMIC_BITSET_CONSTEXPR20 bool operator<( const bit_iterator_base< Iter > & lhs, const bit_iterator_base< Iter > & rhs );
+    template< typename Iter >
+    friend BOOST_DYNAMIC_BITSET_CONSTEXPR20 difference_type operator-( const bit_iterator_base< Iter > & lhs, const bit_iterator_base< Iter > & rhs );
 
 protected:
     BOOST_DYNAMIC_BITSET_CONSTEXPR20 void increment();