Move the prototype for seahorse_pkcs11_backend_initialize into the
header file, so C code generated from Vala sources will use it.
This avoids an implicit function declaration and build failures
with future compilers.

Submitted upstream:

  <https://gitlab.gnome.org/GNOME/seahorse/-/merge_requests/214>

diff --git a/pkcs11/seahorse-pkcs11-backend.c b/pkcs11/seahorse-pkcs11-backend.c
index b7ea21e1fdb1df7c..31d7cb81510a003b 100644
--- a/pkcs11/seahorse-pkcs11-backend.c
+++ b/pkcs11/seahorse-pkcs11-backend.c
@@ -43,8 +43,6 @@ enum {
 	PROP_LOADED,
 };
 
-void  seahorse_pkcs11_backend_initialize (void);
-
 static SeahorsePkcs11Backend *pkcs11_backend = NULL;
 
 struct _SeahorsePkcs11Backend {
diff --git a/pkcs11/seahorse-pkcs11-backend.h b/pkcs11/seahorse-pkcs11-backend.h
index ddcfd2fcc54c43e9..65efbd0a04ba9700 100644
--- a/pkcs11/seahorse-pkcs11-backend.h
+++ b/pkcs11/seahorse-pkcs11-backend.h
@@ -43,4 +43,6 @@ SeahorsePkcs11Backend *  seahorse_pkcs11_backend_get           (void);
 GcrCollection *          seahorse_pkcs11_backend_get_writable_tokens (SeahorsePkcs11Backend *self,
                                                                       gulong with_mechanism);
 
+void  seahorse_pkcs11_backend_initialize (void);
+
 #endif /* SEAHORSE_PKCS11_BACKEND_H_ */