agent: Fix last commit.

* configure.ac: Check ucred.h as well as sys/ucred.h.
* agent/command-ssh.c: Add inclusion of ucred.h.

It was T2981, adding ucred.h for Solaris. We also need sys/ucred.h
for FreeBSD and macOS.

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