guuid: fix shift operation to parse hex string in uuid_parse_string()

The actually parsed `@bytes` are not used because the only caller
does not provide an output parameter to request them. So this bug had
no effect in practice.
diff --git a/glib/guuid.c b/glib/guuid.c
index f26343e..5368465 100644
--- a/glib/guuid.c
+++ b/glib/guuid.c
@@ -113,7 +113,7 @@
       if (hi == -1 || lo == -1)
         return FALSE;
 
-      bytes[i++] = hi << 8 | lo;
+      bytes[i++] = hi << 4 | lo;
     }
 
   if (uuid != NULL)