agent: Set assuan system hooks before call of assuan_sock_init.

* agent/gpg-agent.c (initialize_modules): Move assuan_set_system_hooks.
(main): ... here, just before assuan_sock_init.

In Assuan, global variable SOCK_CTX is used internally, which is
initialized by assuan_sock_init. When initialized, system hooks
are copied into SOCK_CTX structure. Thus, system hooks should
be set, before the call of assuan_sock_init.

• GnuPG-bug-id: T3378
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>