[perl-Gtk2-Unique] Fix compilation warnings with unique_message_data_get

[Emmanuel Rodriguez <erodriguez src gnome org>]

commit 8ac892efdf480efbca75dc1729fc9aa45708618e
Author: Emmanuel Rodriguez <emmanuel rodriguez gmail com>
Date:   Fri Feb 24 22:05:13 2017 +0100

    Fix compilation warnings with unique_message_data_get
    
    xs/UniqueMessageData.xs:44:50: warning: passing argument 2 of ‘unique_message_data_get’ from incompatible 
pointer type [-Wincompatible-pointer-types]
       string = unique_message_data_get(message_data, &length);

 xs/UniqueMessageData.xs |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/xs/UniqueMessageData.xs b/xs/UniqueMessageData.xs
index 7c58710..eeee886 100644
--- a/xs/UniqueMessageData.xs
+++ b/xs/UniqueMessageData.xs
@@ -38,7 +38,7 @@ SV*
 unique_message_data_get (UniqueMessageData *message_data)
        PREINIT:
                const guchar *string = NULL;
-               gint length = 0;
+               gsize length = 0;
                
        CODE:
                string = unique_message_data_get(message_data, &length);