agent: Fix coercion for pinentry_pid handling.

* agent/call-pinentry.c (start_pinentry): Don't use pid_t.

When pid_t is 64-bit integer and unsigned long is 32-bit,
it never matches, because left hand side does not fill
upper 32-bit.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>