Fix mismatched DMAPMetaDataMap struct typedef

A mismatched struct typedef in dmap-share.h confuses GObject introspection
into producing an error when the original struct is defined. The typedef
of DMAPMetaDataMap asks for struct _DMAPMetaDataMap, but the actual
struct definition says struct DMAPMetaDataMap. gir will complain about
namespace conflicts and abort a jhbuild build as a result.

Thanks to Pietro Gagliardi and gregier in irc.gimp.net/#gtk+ who helped
Pietro.

Signed-off-by: W. Michael Petullo <mike@flyn.org>