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

[Emmanuel Rodriguez <erodriguez src gnome org>]

commit 4c2b2ba2e0cb6fb535581761b0120d133be83dcb
Author: Emmanuel Rodriguez <emmanuel rodriguez gmail com>
Date:   Fri Feb 24 21:54:53 2017 +0100

    Fix compilation warnings with unique_command_from_string
    
    xs/UniqueApp.xs: In function ‘XS_Gtk2__UniqueApp_send_message_by_name’:
    xs/UniqueApp.xs:323:19: warning: implicit declaration of function ‘unique_command_from_string’ 
[-Wimplicit-function-declaration]
          command_id = unique_command_from_string(app, command_name);

 xs/UniqueApp.xs |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/xs/UniqueApp.xs b/xs/UniqueApp.xs
index cd38bbc..93ee4cd 100644
--- a/xs/UniqueApp.xs
+++ b/xs/UniqueApp.xs
@@ -2,6 +2,7 @@
 #include <gperl_marshal.h>
 
 G_CONST_RETURN gchar * unique_command_to_string (UniqueApp *app, gint command);
+gint unique_command_from_string (UniqueApp *app, const gchar *command);
 
 static void
 perl_unique_app_marshall_message_received (