buffer: fix iterator_impl visibility through typedef

buffer: fix iterator_impl visibility through typedef

The following program doesn't compile because of symbol visibility issues.
While bufferlist::iterator is a class implementation with visibility specified,
it is unclear after google-fu how to do the same through typedef.

int main()
{
 ceph::bufferlist bl;
 ceph::bufferlist::const_iterator it = bl.begin();
 (void)it;
 return 0;
}

[nwatkins@bender ~]$ g++ -Wall -std=c++11 -Iinstall/include -Linstall/lib -o test test.cc -lrados
/tmp/cciR9MUj.o: In function `main':
test.cc:(.text+0x43): undefined reference to `ceph::buffer::list::iterator_impl<true>::iterator_impl(ceph::buffer::list::iterator const&)'
/usr/bin/ld: test: hidden symbol `_ZN4ceph6buffer4list13iterator_implILb1EEC1ERKNS1_8iteratorE' isn't defined
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
(cherry picked from commit 16bc3e59325d4057d36cd63a57111ab36fbc50fd)