Mark some structures in the library interface with visibility=default attribute.

As of gcc 4.6, there are new warnings from -Wattributes along the lines of:

	warning: ‘_notmuch_messages’ declared with greater visibility
	than the type of its field ‘_notmuch_messages::iterator’
	[-Wattributes]

To squelch these, we decorate all such containing structs with
__attribute__((visibility("default"))). We take care to let only the
C++ compiler see this, (since the C compiler would otherwise warn
about ignored visibility attributes on types).