This was found by a third-party testsuite, which complained because we
only define get_deleter(const __shared_ptr<T,L>&) and not the corect
signature.

	* include/bits/shared_ptr.h (get_deleter): Add overload matching
	standard signature.
	* include/bits/shared_ptr_base.h (__shared_ptr): Declare new
	get_deleter overload as a friend.
	* testsuite/20_util/shared_ptr/misc/get_deleter.cc: New.

Tested powerpc64le-linux, committed to trunk.