socket: Don't call pre/post_syscall for bind.

* src/assuan-socket.c (_assuan_sock_bind): Don't call
_assuan_pre_syscall and _assuan_post_syscall.

In the current use cases in GnuPG, the socket is bound *before*
calling npth_init and gpgrt_set_syscall_clamp, which results no
syscall clamping. It's OK for bind to use with no syscall clamping as
it is not blocked longer.

• Fixes-commit: 703b4100a4162216314d3489fa9d3160c3aadeb5
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>