rsvg: Also accept </svg:svg> as ending tag

Some SVG files created by inkscape use that, such as:
https://gitlab.gnome.org/GNOME/pitivi/uploads/8dd8d9d988b5eb6cc38f871196caac6f/Titel-Tafel3.2_anim.svg

This is a first patch for https://bugzilla.gnome.org/show_bug.cgi?id=796909
but it is not enough to support animated gifs.
diff --git a/ext/rsvg/gstrsvgdec.c b/ext/rsvg/gstrsvgdec.c
index e50a897..1cf7ea0 100644
--- a/ext/rsvg/gstrsvgdec.c
+++ b/ext/rsvg/gstrsvgdec.c
@@ -320,6 +320,11 @@
       size = i + 6;
       break;
     }
+    if (memcmp (data + i, "</svg:svg>", 10) == 0) {
+      completed = TRUE;
+      size = i + 10;
+      break;
+    }
   }
 
   if (completed) {